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Chemosphere 227 (2019) 681e702

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Chemosphere

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Emerging sustainable technologies for remediation of soils andgroundwater in a municipal solid waste landfill site – A review

Jianshe Ye a, Xiao Chen a, Chao Chen a, Bate Bate b, *

a Graduate Research Assistant, Institute of Geotechnical Engineering, College of Civil Engineering and Architecture, MOE Key Laboratory of Soft Soils andGeoenvironmental Engineering, Zhejiang University, Chinab Institute of Geotechnical Engineering, College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, China

h i g h l i g h t s

� Landfills contaminants are grouped into COD, inorganic matter and heavy metals.� A status quo of existing technologies were thoroughly reviewed.� A design chart was developed, verified by a few case studies.� Future trends of technical innovation and challenges were identified.

a r t i c l e i n f o

Article history:Received 1 February 2019Received in revised form5 April 2019Accepted 6 April 2019Available online 10 April 2019

Handling Editor: Daniel CW Tsang

Keywords:RemediationLandfillPermeable reactive barrierElectrokineticMicro and nanobubblesSolubilizing agent

* Corresponding author.E-mail address: [email protected] (B. Bate).

https://doi.org/10.1016/j.chemosphere.2019.04.0530045-6535/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

Remediation of soils and groundwater in a municipal solid wastes (MSW) landfill site emerges as a globalchallenge to the living environment on earth with significant market potential. Unlike contaminants inan industry or agricultural site, contaminants from MSW landfills are diverse, primarily consisting ofchemical oxygen demand (COD), inorganic matter (ammonia-nitrogen, nitrate-nitrogen, total phos-phorus) and heavy metals. This renders new challenges to remediation contaminants of differentcharacters altogether. A status quo of existing technologies, including permeable reactive barriers,electrokinetic remediation, microbial remediation, and injection of either solubilizing agents or micro ornanobubbles were thoroughly reviewed, with an emphasis on removal efficiency based on existingprojects at lab, pilot or field scales. A design chart tailored for the remediation of a landfill contaminatedsite was developed, verified by a few case studies, which supplement the chart. Future trends of technicalinnovation (such as multi-layer permeable reactive barriers (PRBs)) and challenges (such as flow pattern)were identified.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

Soil and groundwater pollution are a global challenge. Morethan 5 million contaminated sites need to be remediated, amongwhich 20 million hectares of sites are contaminated by heavymetals (Wuana and Okieimen, 2011). Many municipal solid waste(MSW) landfills are either historically unregulated (unlined),poorly constructed, or aged, which makes them a major source ofpollution. Due to degradation of wastes, microbial metabolism,rainfall, and groundwater intrusion in landfills, leachate is gener-ated, but is often not contained, diverted, collected, or treated. This

leads to contamination of the soil, groundwater, air and the vadosezone surrounding landfills (Liu et al., 2018). Such negligence sub-sequently poses severe risks to human health (sometimes evencarcinogenic, leukemic, or leading to abortion), and to agricultureand the ecological system.

Such risks have raised increasing worldwide attention over thepast decades and have given rise to a booming global market forsite remediation. In China, 1.01 million square kilometers of sitesexceed environmental quality standard for soils (GB15618-1995)(MEP-PRC and MLR-PRC, 2014). Moreover, about 1600 MSW land-fills and 27000 simple landfills are prone to leachate leakage(MHURD-PRC, 2018); Among the 188 solid waste landfills surveyed(1351 survey sites), 21.3% exceeded the GB15618-1995 standard(MEP-PRC and MLR-PRC, 2014). The Forward Industrial Research

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Institute (Qin and Zhang, 2018) predicted that Chinese market ofcontaminated sites remediation reached 682.1 billion Chineseyuan, among which 21.6 billion Chinese yuan was for solid wastelandfills. Europe had identified 80,000 contaminated sites by 2007(European Environmental Agency (EEA), 2007) and 2.5 millionpotentially contaminated sites (PCS) by 2013, among which 38%were caused by MSW landfills (Panagos et al., 2013). The remedi-ation market was estimated to be 6 billion Euro annually (Panagoset al., 2013). In United States, it was estimated that about 294,000contaminated sites require treatment from 2004 to 2033, whichwill cost 209 billion USD (US Environmental Protection Agency(USEPA), 2004) and cost a total of 650 billion USD is expected inthe next 30e35 years (Naidu and Birke, 2014). Australia has morethan 50,000 confirmed contaminated sites and 160,000 PCS. Theannual treatment cost is estimated to exceed 3 billion USD (Heet al., 2015; Naidu and Birke, 2014). Japan has over 500,000 PCS,and their annual treatment cost is about 3 billion USD by 2010(Naidu and Birke, 2014). A summary of the worldwide status quo ofcontaminated sites and the remediation market is summarized inTable 1.

The complicated and spatiotemporally heterogeneous contam-inants fromMSW landfill leachates can be grouped into four majorcategories, namely, dissolved organic matter (quantified by COD),inorganic contaminants (including ammonium-nitrogen, nitrate-nitrogen, total phosphorus), heavy metals (Fe2þ, Mn2þ, Cd2þ, Cr(VI), Cu2þ, Pb2þ, Ni2þ and Zn2þ), and the broad range of xenobioticorganic compounds (Kjeldsen et al., 2002; Han et al., 2016a,b).

Features of young MSW landfills leachate (<10 years old) differfrom old MSW landfills (Renou et al., 2008). The young landfills aredivided into two phases: aerobic degradation of microorganisms(typically short duration) and anaerobic degradation of microor-ganisms (acid production); while an old landfill is divided into twophases: themethanogenic phase (microbial anaerobic degradation)and the stabilization phase (Kjeldsen et al., 2002; Renou et al.,2008). Generally, pH value increases and the COD concentrationdecreases while the ammonia concentration usually does notdecrease in the leachate as landfill ages, which is the long-termcontaminant component in the leachate (Kjeldsen et al., 2002).Metal solubility decreases as pH increases, so the concentration ofheavy metals is generally very low for old landfills (Ehrig, 1983;Kjeldsen et al., 2002; Kulikowska and Klimiuk, 2008). On theother hand, concentrations of total phosphorus, calcium, magne-sium, heavymetal, BTEX are influenced by season of the year, ratherthan landfill age (Kulikowska and Klimiuk, 2008).

Features of the leachate components from six countries or

Table 1Status quo of global polluted sites and remediation markets.

Country Identified and potential contaminated sites V

Globally >5 million sites, 20 million ha of land are contaminated by heavy metal e

China 1.01 million square kilometers of soil $101.6 billion

1600 MSW landfills and 27000 simple landfills e

Europe >80000 pollution sites e

342,000 identified contaminated areas and 2.5million PCSa

$6.7 billion annually

UnitedStates

294000 contaminated sites (including identifiedsites and PCS)

e

Australia >50000 pollution sites e

160,000 PCS >$3 billion per annumJapan 500,000 þ PCS $1.2 billionþ, timefram

unspecifiedBrazil 5942 identified contaminated areas e

a MEP-PRC, Ministry of Environmental Protection of the People's Republic of China; MPRC, Ministry of Housing and Urban-Rural Development-People's Republic of China; PCS

regions are summarized in Fig. 1, which provide the following in-formation: 1) Concentrations of COD and inorganic pollutants inleachates are high, while the concentration of heavy metals isgenerally low (expect in Beijing). 2) The COD concentration of thenew landfill is much higher than that of the old landfill.

The objective of this paper is to identify existing technologiesthat are suitable for sustainable remediation of solid waste sites, toprovide guidance for future work, and to identify challenges. In thefollowing sections, Part 1 summarizes existing and emergingremediation methods for landfills contaminated sites; Part 2 pro-vides a design chart, which suggests the proper remediationtechnology for a specific hydrogeological condition, and a few casestudies as examples; Part 3 identifies challenges to sustainableremediation methods. Then a summary is given. The layout of thecontent is as follows:

2. Part 1. remediation technologies for contaminated MSWsites

Several remediation technologies have been developed over thepast decades to remediate soil and groundwater contaminated bypractices of chemical engineering, petroleum engineering, agri-culture or mining operations. These remediation technologiesinclude permeable reactive barriers (PRBs), electrokinetic (EK)technology, biologically enhanced degradation, solubilizing agents,micro bubbles, and nanobubbles. A detailed examination of thesetechnologies is provided below, which can be used to assist inidentifying techniques suitable for sustainable remediation ofcontaminated municipal solid waste sites.

2.1. Permeable reactive barrier

A permeable reactive barrier (PRB), as defined by the U.S.Environmental Protection Agency (US EPA, 1998; Powell et al.,1998), is “an emplacement of reactive materials in the subsurfacedesigned to intercept a contaminant plume, provide a flow paththrough the reactive media, and transform the contaminant(s) intoenvironmentally acceptable forms to attain remediation concen-tration goals down gradient of the barrier.” Approximately 200 PRBapplications were constructed in Europe, North American andAustralia from 1994 to 2005, among which 120 were zero valenceiron-based (with 83 applications in full-scale) (InterstateTechnology and Regulatory Council (ITRC), 2005; Gillham et al.,2010; Liu et al., 2015a,b). The annual number of published articleson PRB increases steadily from 2000 to 2018 (Fig. 2). In China, most

alue of Current Market Future Market Potential Reference

e Wuana and Okieimen (2011)

Unassessed a MEP-PRC and MLR-PRC. (2014); Qin andZhang (2018)

e a MHURD-PRC (2018)e He et al. (2015)e Panagos et al. (2013)

209 billion USD from 2004 to2033

a US EPA (2004)

e He et al. (2015)Unassessed Naidu and Birke (2014)

e Estimated to grow to $3 billionby 2010

Naidu and Birke (2014)

e Thom�e et al. (2018)

LR-PRC, Ministry of Land and Resources of the People's Republic of China; MHURD-, potentially contaminated sites; US EPA (U.S. Environmental Protection Agency).

Fig. 1. Statistics on chemical characteristics of landfill leachate from selected countries and regions. All landfills are old landfills, except for those in Hong Kong SAR (younglandfills) and in Beijing (including both young and old landfills). The height of the histogram represents the average concentration value, and the upper and lower pointscorrespond to the maximum and minimum concentration values, respectively. Note: the primary and secondary ordinates are logarithmic coordinates.

Fig. 2. Number of articles under the topic of “permeable reactive barrier” and “zero valence iron permeable reactive barrier” from 2000 to 2018 (Data retrieved from Web ofScience).

J. Ye et al. / Chemosphere 227 (2019) 681e702 683


PRB research is still at lab scale, with a few pilot PRB applicationsreported in Shenyang (Hou et al., 2014; Li et al., 2014), Baotou,Jiaozuo and Changsha.

Commonly-used reactive materials in a PRB are zero valenceiron, activated carbon and zeolite, while other reactive materialsinclude steel slag, scrap rubber, sponge iron, and medical stone(Tables 2e5). Reversible absorption and irreversible redox reaction

Table 2Removal rates of COD.

Remover Removal mechanism Contaminant Conditi

Atomized slag filtration andabsorption

waste landfillleachate in Korea

pump rpore vo

Waste rubber chemical adsorption Laboratory preparedgroundwatercontaminated byleachate

Colloid200mLfrequen

different ratios of ZVI/Zeo/AC chemical adsorption Landfill leachatefrom Guangdong,China

flow ra

Clinoptilolite strong adsorptioncapacity and containsexchangeable cationswith ion exchangecharacteristics

40-year-old Belgianlandfill (1967)leachate 10mL

electric51± 4m0.16e1

Steel slag and zeolite high porosity andadsorption capacity

Laboratory preparedgroundwatercontaminated byleachate

flow ra

Waste rubber chemical adsorption Laboratory preparedgroundwatercontaminated byleachate

flow ra

Bentonite and activated carbon(3%) sample/solutionratio¼ 3 g/50ml

absorb the complexcomponents of variousorganic molecules

Anaerobic old-fashioned garbagesimulation columnfor leachaterecirculationtreatment

stirredmin, ststand f

quartz 34.78%, ZVI 43.48% andzeolite 21.74%

high porosity andadsorption capacity

Leachate fromShibeiling Landfill,Changchun, China

(1) zeolite (2) ceramsite (3)activated carbon (4) fly ash(5) anthracite (6) 80% slagand 20% ZVI (7) 80% activatedcarbon and 20% ZVI

Fe0 reduction;adsorption;biodegradation

Landfill Leachatediluted 45 times(Shijiazhuang, China)

flow ra

(1) zeolite (2) ceramsite (3)activated carbon (4) slag 50%and ZVI 50% (5) 50% activatedcarbon and 50% ZVI


Landfill Leachatediluted 50 times(Shijiazhuang, China)

flow ra

Clinoptilolite (SiO4/AlO4) strong adsorption andcontains exchangeablecations with ionexchangecharacteristics

Pig farm wastewater pH8, h375mL

(1) steel slag (2) zeolite (3)anthracite (4) steel slag,zeolite, anthracite mixedfiller (5) steel slag, zeolite,anthracite combined filler


Coal mine pit waterfrom Inner Mongolia,China

1-4 sanflow rahydrau(m2d);rate 0.35 sandflow rahydrau(m2d)

(1) sand 40% and ZVI 60%; (2)sand 34.78%, ZVI 43.48% andactivated carbon 21.74%; (3)sand 40%, ZVI 40% and zeolite20%;


Leachate formed bysoaking garbage(Shibeiling,Changchun, China)

seepage85 cm

are the primary remediation mechanisms in a PRB (Lu et al., 2006).Replacement of the reactive medium is required at intervals tocounter performance degradation, saturation, or depletion of ma-terials. Most existing PRBs contain only one type of reactive ma-terial, which also targets removing contaminants of the same group(such as those leaked from a gas station). Recently, multi-layer PRBhas been the research focus for remediation of complex

on of removal Type ofexperiment

Initialconcentrationofcontaminant

Removalefficiency(%)

Removaltime

Reference

ate 0.8mL/min; 10lumes

column 642mg/L 40 Chunget al.(2007)

al particles 0.5 g/, oscillationcy 150 r/min,pH7

lab 250mg/L 44.3 36 h Jiao and Di(2016)

te 0.5mL/min column 108.72mg/L 55.8 576 h Zhou et al.(2014)

al conductivityS/cm,<flow rate

.05m/d

column 389± 36mgN/L

51e72 127d vanNootenet al.(2008)

te 40e100 cm/d column 250mg/L 61.2 48d Di et al.(2014)

te 0.45e0.85m/d column 250mg/L 73.2 Di et al.(2013a,b,c)

at a speed of 200 r/irred for 2 h, and letor 1 h

batch 1100mg/L 77.3 3 h Ji et al.(2012)

lab 1027.1mg/L 80.5 Dong et al.(2009)

te 80e150 cm/d column 42.70mg/L 82.22e88.11

40d Cui et al.(2010)

te 90e150 cm/d column 45.5mg/L 82.36e88.17

65d Cui et al.(2010)

ydraulic load/h

column 1011mg/L 84 24 h Song et al.(2011)

d box: 8 h, averagete 0.76 L/h,lic load 0.0704m3/16 h, average flow8 L/h;box: 8 h, averagete 0.86 L/h,lic load 0.239m3/

lab 15mg/L 85.78 Di et al.(2013b)

e velocity 45/d

column 1000.6mg/L 90(80,90,70)

Dong et al.(2003)

Table 3Removal rate of ammonia-nitrogen.

Remover Removal mechanism Contaminant Condition of removal Type ofexperiment



Removaltime

reference

ZVI/Zeolite/AC at differentratios

chemical adsorption Landfill leachatefrom Guangdong,China

flow rate 0.5mL/min column TN 35.47mg/L 70.8 Zhouet al.(2014)

(1) sand 40% and ZVI 60%;(2) sand 34.78%, ZVI43.48% and activatedcarbon 21.74%; (3) sand40%, ZVI 40% and zeolite20%

Fe0 reduction; adsorption;biodegradation

Leachate formedby soakinggarbage(Shishiling,Changchun,China)

seepage velocity 45e85 cm/d column 45.41mg/L 78e91 Donget al.(2003)

(1) quartz sand 44.4% andZVI55.6%; (2) quartz sand75% and ORC 25%

Fe0 reduction; adsorption;biodegradation

Leachate formedby soakinggarbage(Shishiling,Changchun,China)

flow rate 70e80 cm/d lab 60.35mg/L 85 Donget al.(2004)

(1) steel slag (2) zeolite (3)anthracite (4) steel slag,zeolite, anthracite mixedfiller (5) steel slag,zeolite, anthracitecombined filler


Coal mine pitwater, InnerMongolia, China

1st-4th sand box:8 h, averageflow rate 0.76 L/h, hydraulic load0.0704m3/(m2d); 16 h, averageflow rate 0.38 L/h;5th sand box; 8 h, average flowrate 0.86 L/h, hydraulic load0.239m3/(m2d)

lab 200mg/L 88.8 Di et al.(2013b)

ZVI/Zeolite/AC mixtures atdifferent ratio

chemical adsorption Landfill leachatefrom Guangdong,China

flow rate 0.5mL/min column 29.77mg/L 89.2 Zhouet al.(2014)


Laboratorypreparedwastewater

The particle size, dosage andoscillation frequency of zeoliteand steel slag are: 0.297e0.595mm, 10 g/100mL, 150 r/min

batch 50mg/L 91.7 2 h Di et al.(2013c)

Clinoptilolite (SiO4/AlO4) strong adsorption andcontains exchangeablecations with ion exchangecharacteristics

Pig farmwastewater

pH8, hydraulic load 375mL/h column 700mg/L 96 24 h Songet al.(2011)


Laboratorypreparedcontaminatedgroundwater

flow rate 40e100 cm/d column 25mg/L 96.3 48d Di et al.(2014)

quartz 34.78%, ZVI 43.48%and zeolite 21.74%


Leachate fromShibeiling Landfill,Changchun, China

lab 60.4mg/L 97.4 Donget al.(2009)

Clinoptilolite strong adsorptioncapacity and containsexchangeable cations withion exchangecharacteristics

Synthesizedgroundwater

flow rate 15mL/min pilot-scale 5.29e10.80mg/L

99 Huanget al.(2015)

zeolite sorption and ion exchange the Hun River inShenyang

hydraulic conductivity7� 10�3m/s

pilot-scale 2e10mg/L 90 Hou et al.(2014)

zeolite sequential nitrification,adsorption, anddenitrification

River water fromHun River,Shenyang, China

flow rate 1m/d pilot-scale 0.3e36.6mg/L 90 382d Li et al.(2014)


contaminants in solid waste landfills and in wastes from pharma-ceutical industries. (Yang et al., 2013; Pawluk and Fronczyk, 2015;Poło�nski et al., 2017; Kumarasinghe et al., 2018; Pawluk et al., 2019).

The efficiencies of PRB reactive materials on removing COD,ammonia nitrogen, nitrate-nitrogen, total phosphorus and heavymetals from different studies are summarized in Tables 2e6 andanalyzed as follows.

The removal rate of COD in contaminated water of a landfillranges from 44.3% to 90% (Table 2), with the highest removal rate(90%) obtained with reactive materials of a mixture of activatedcarbon and zero valence iron (Dong et al., 2003). Other notablereactivematerials for COD removal are amixture of anthracite, steelslag, and zeolite (85.78% removal efficiency via adsorption byintermolecular force) (Di et al., 2013a,b,c), and clinoptilolite (84%removal efficiency via adsorption by the strong dispersive force and

electrostatic attraction of the surface) (Song et al., 2011).Removal rate of ammonia nitrogen ranges from 70.8% to 99%

(Table 3). In the laboratory, a mixture of zeolite and zero-valenceiron yielded the highest removal rate of 97.4% (Dong et al., 2009).Zeolite adsorbs NHþ

4 � N, while ZVI react with NHþ4 � N by redox

reaction. Other notable reactive materials for NHþ4 � N removal

include clinoptilolite (96% removal efficiency via the strongdispersive force and electrostatic attraction of the surface) (Songet al., 2011), and mixtures of zero-valence iron, zeolite and acti-vated carbon at different proportions (89.2% removal efficiency viaadsorption by intermolecular force) (Zhou et al., 2014).

Removal rate of nitrate-nitrogen, primarily obtained at lab scale,ranges from 76.1% to 99.87% (Table 4). The highest removal rate ofNO�

3 � N (99.87%) was achieved by loading modified nano-iron

Table 4Removal rate of nitrate-nitrogen.



Removalefficiency (%)

Removaltime

reference

(1) sand 40% and ZVI 60%; (2) sand 34.78%, ZVI43.48% and activated carbon 21.74%; (3)sand 40%, ZVI 40% and zeolite 20%

Fe0 reduction; adsorption; biodegradation Leachate fromShibeilingLandfill,Changchun,China

hydraulic conductivity 45e85 cm/d column 48.24mg/L 85 Donget al.(2003)

(1) ZVI þ medium coarse sand (2)AC þ medium coarse sand (3)sawdust þ medium coarse sand (4)ZVI þ AC þ medium coarse sand (3:1:6) (5)ZVI þ sawdust þ medium coarse sand

Adsorption Laboratorypreparation ofgroundwatercontaminated

column 80mg/L 76.1 10d Liu et al.(2017)

13 groups of tests, sawdust þ ironpowder > sawdust aftercomposting > sawdust > iron powder

Laboratorypreparation ofgroundwatercontaminated

column 80 Wanget al.(2008)

(1) quartz sand 44.4% and ZVI55.6%;(2) quartz sand 75% and ORC 25%

Fe0 reduction; adsorption; biodegradation Leachate fromShibeilingLandfill,Changchun,China

hydraulic conductivity 70e80 cm/d lab 48.24mg/L 80 Donget al.(2004)

nZVI 0.5 g Fe0 reduction; adsorption Rotating speed 250 r/min lab 125mg/L 90 Li et al.(2006)

Straw compost humus soil: fine sand 1:10 or1:50

Anaerobic respiration Laboratorypreparedcontaminatedgroundwater

T¼ 25 �C, pH¼ 7e8 lab 150mg/L 91.8 15d Yang andWu(2016)

n-ZVI and carbon composite 5:2 Adsorption Laboratorypreparation ofgroundwatercontaminated

Stirring speed 500 r/min lab 60mg/L 94.3 Tanget al.(2016)

(1) sponge iron (2) sponge iron þ AC singlelayer medium 1:1 (3) spongeiron þ AC þ Zeo single layer medium 1:1:1(4) sponge iron þ AC1:1 mixing and Zeodouble layer media 2:1

After the reaction, many oxides and hydroxides wereformed on the surface of the sponge iron, and themicropores were severely blocked, which hinderedthe further reaction of the sponge iron with NO-3-N.Oxidation


flow rate 2.6 cm/h column 30mg/L 91.1,94.4,87.9,91.0 8d Zhu et al.(2017)

0.15e0.42 mm ZVIþ0.15 mm AC1:1 Fe0 reduction; adsorption simulatecontaminatedgroundwater

pH6.9e7.1, flow rate 50e80 cm/d lab 20mg/L 95 Menget al.(2012)

Medical stone, iron filings Medical stone adsorbs and the role of Iron filings areadsorption and chemical denitrification


Flow rate 0.18 L/d column 20mg/L 97.3 40d Di et al.(2016)

Modified nano iron/carbon 5:2 (1.5 g) Adsorption Laboratorypreparedcontaminatedgroundwater

flow rate 1mL/min, pH7 column 40mg/mL 99.9 100min Gao et al.(2016)

cotton as carbon source and reaction medium using cotton as a carbon source and reactionmedium to remove nitrate from groundwater


T¼ 25± 1 �C lab 22.6mg/L 100 Jin et al.(2004)

acid-washed waste steel scrap convert NO3� to NO2

�, NH3, and NH4þ leachate

simulantreactors were sealed withsiliconlined, gray-butyl septa, andaluminum crimps. Reactors wereshaken on a reciprocating shaker(150 rpm, 25 �C)

batch 10mg/L 100 7 h Oh et al.(2007)

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686

Table 6Removal rates of heavy metals.




Removaltime

Reference

Fe Waste rubber chemical adsorption Laboratory preparedcontaminatedgroundwater

flow rate 0.45e0.85m/d

column 50mg/L 95.2 30d Di et al.(2013a,b,c)

Waste rubber chemical adsorption Laboratory preparedcontaminatedgroundwater

Colloidal particles 0.5 g/200mL, oscillationfrequency 150 r/min,pH7



Laboratory preparedwastewater

The particle size,dosage and oscillationfrequency of zeolite andsteel slag are: 0.297e0.595mm, 10 g/100mL, 150 r/min.



Laboratory preparedcontaminatedgroundwater


Mn Waste rubber chemical adsorption Laboratory preparedcontaminatedgroundwater

Colloidal particles 0.5 g/200mL, oscillationfrequency 150r/min,pH7


sand 40%, ZVI 40% andzeolite 20%


Leachate formed bysoaking garbage inShishiling, Changchun,China

seepage velocity 45e85 cm/d

column 60.89mg/L 90 Dong et al.(2003)




(continued on next page)

Table 5The removal rate of total phosphorus.


Initialconcentration ofcontaminant


Removaltime

Reference

(1) steel slag (2) zeolite(3) anthracite (4)steel slag, zeolite,anthracite mixedfiller (5) steel slag,zeolite, anthracitecombined filler


Coal mine pit water,Inner Mongolia,China

1st-4th sand box: 8 h, averageflow rate 0.76 L/h, hydraulicload 0.0704m3/(m2d); 16 h,average flow rate 0.38 L/h;5th sand box: 8 h, average flowrate 0.86 L/h, hydraulic load0.239m3/(m2d)

lab 50mg/L 96.1 Di et al.(2013b)

Clinoptilolite (SiO4/AlO4)

strong adsorption andcontains exchangeablecations with ionexchangecharacteristics

Pig farm wastewater pH8, hydraulic load 375mL/h column 50mg/L 97 24 h Songet al.(2011)

aid treated Basicoxygen furnace slag

chemical andgeometricalheterogeneities;abundant calcium andaluminum ions whichenhance the processesof adsorption andchemical precipitationfor PO4

3-

synthesized Artificialwastewater

pH¼ 7.0e7.2, 20 �C batch 10mg/L 98 1 h Xue et al.(2009)

Modified steel slagobtained by mixingsteel slag andaluminum hydroxidein a mass ratio of 4:3and calcined at700 �C for 2 h

Chemical precipitationand ligand exchange


pH¼ 4e8, 30 �C batch 2mg/L 99 2 h Duanet al.(2012)

Converter slag reacted (formation ofprecipitate) with metalcations (Al3þ, Fe2þ,Fe3þ, Ca2þ etc.), whichare commoncomponents ofconverter slag insolution

leachate simulant reactors were sealed withsiliconlined, gray-butyl septa,and aluminum crimps. Reactorswere shaken on a reciprocatingshaker (150 rpm, 25 �C)

batch 10mg/L 100 20min Oh et al.(2007)


Table 6 (continued )




Removaltime

Reference



The particle size,dosage and oscillationfrequency of zeolite andsteel slag are: 0.297e0.595mm, 10 g/100mL, 150 r/min


(1) steel slag (2) zeolite(3) anthracite (4) steelslag, zeolite, anthracitemixed filler (5) steelslag, zeolite, anthracitecombined filler


Inner Mongolia coalmine pit water

1st-4th sand box: 8 h,average flow rate0.76 L/h, hydraulic load0.0704m3/(m2d); 16 h,average flow rate0.38 L/h;5th sand box: 8 h,average flow rate0.86 L/h, hydraulic load0.239m3/(m2d)

lab 1.5mg/L 98.98 Di et al.(2013b)

quartz 34.78%, ZVI43.48% and zeolite21.74%


Leachate from ShibeilingLandfill, Changchun,China

lab 13.8mg/L 99.6 Dong et al.(2009)

Zn sand 40%, ZVI 40% andzeolite 20%;


Leachate formed bysoaking garbage inShishiling, Changchun,China

seepage velocity 45e85 cm/d

column 70.35mg/L 80 Dong et al.(2003)





calcite, vegetal compostand sewage sludge andFe0

Calcite was used toraise pH and precipitatemetals as (oxy)hydroxides andcarbonates

Groundwatercontaminated with acidmine drainage, Spain

hydraulic conductivity:10e400m/day

field 15mg/L 95 Gibertet al.(2013)

acid-washed wastesteel scrap

adsorption of heavymetals by iron oxide onthe acid-washed wastesteel scrap surface

Leachate simulant reactors were sealedwith siliconlined, gray-butyl septa, andaluminum crimps.Reactors were shakenon a reciprocatingshaker (150 rpm, 25 �C)

10.2mg/L 98 6 h Oh et al.(2007)

acid-washed ZVI/ZVAl¼ 80 g/40 g

aluminum is aneffective electrondonor, actingas a strong reductant toremove heavy metalions

wastewater containingheavy metal

pH¼ 5.4; flowrates¼ 1.0mL/min

column 20mg/L 99.5 300 h Han et al.(2016a,b)

Cr(VI)

Medical stone, Ironfilings

Medical stone adsorbsand the role of Ironfilings are adsorptionand chemicaldenitrification


flow rate 0.18 L/d column KCr2O7 10mg/L

97.7 40d Di et al.(2016)

0.15e0.42 mmZVI þ 0.15 mm AC1:1

Fe0 reduction;adsorption

Contaminatedgroundwater simulant

pH 6.9e7.1, flow rate50e80 cm/d

lab 10mg/L 96 Meng et al.(2013)





13X zeolite andvermiculite

adsorption 50mg/L 78.2 Silva et al.(2017)

Medical stone, Ironfilings

Medical stone adsorbsand the role of Ironfilings are adsorptionand chemicaldenitrification


flow rate 0.18 L/d column 10mg/L 97.7 40d Di et al.(2016)


aluminum is aneffective electrondonor, acting as astrong reductant toremove heavy metalions




Sulfuric acid modifiedfly ash

adsorption (van derWaals force, chemicalbond force, hydrogen

Adding hexavalentchromium togroundwater

pH¼ 2, 25 �C batch 5mg/L 99 3 h Huanget al.(2012)






Removaltime

Reference

bond force andelectrostatic attraction)

Ni ZVI/Zeolite/AC atdifferent ratios

chemical adsorption Landfill leachate fromGuangdong, China

flow rate 0.5mL/min column 0.015mg/L 70.7 576 h Zhou et al.(2014)

ZVI/Pumice granular flow rate 0.5 cm3/min column 5mg/L 99.9 3240 h Calabr�oet al.(2012)






(1) ZVI 240 g (2) lapillus1289 g (3) ZVIþ lapillus

flow velocity 0.4m/d Benchmarkcolumn

50mg/L 99 Madaffariet al.(2017)

Cd acid-washed ZVI/ZVAl¼ 80 g/40 g









Pb ZVI/Zeolite/AC atdifferent ratios

chemical adsorption Landfill leachate fromGuangdong, China

flow rate 0.5mL/min column 0.006mg/L 92.7 576 h Zhou et al.(2014)

pumice, perlite andlime 2:1:2

adsorption flow rate 2.4mL/min 200mg/L 99.9 50d Ranjbaret al.(2017)

Fig. 3. Number of articles under the topic of “electrokinetic” from 2000 to 2018 (Dataretrieved from Web of Science).


onto activated carbon to constitute SM-nano iron/carbon, whichboth enhanced adsorption onto AC and expanded the reaction areaof nZVI (due to its attachment onto the AC surface) (Gao et al.,2016). Other effective reactive materials for nitrate-nitrogenremoval are the combination of sponge iron and activated carbon(94.4% removal efficiency via adsorption) (Zhu et al., 2017) and thecombination of medical stone (adsorption) and iron filings (97.34%removal efficiency via adsorption and chemical denitrification) (Diet al., 2016).

Relatively few studies have been conducted on total phosphorus(TP) contaminants (Table 5). The notable reactive materials fornitrate-nitrogen removal are a combined packing of anthracite,steel slag, and zeolite (96.08% removal efficiency via adsorption byintermolecular force) (Di et al., 2013a,b,c), and clinoptilolite (97%removal efficiency via adsorption) (Song et al., 2011). Other reactivematerials reported for removing total phosphorus include acid-treated basic oxygen furnace slag (98% removal efficiency viaadsorption and chemical precipitation for PO4

3�(Xue et al., 2009)and modified steel slag (99% removal efficiency via chemical pre-cipitation and ligand exchange) (Duan et al., 2012).

The removal rates of heavy metals are usually high, exceeding95% in many instances (Table 6). Steel slag and zeolite were used toremove iron from leachate-contaminated groundwater at aremoval rate of 99.8% (Di et al., 2014a,b). ZVI and zeolite were usedto remove Mn, Zn, Cd and Cr(VI) in leachate from the ShibeilingLandfill, Changchun, China with removal rates of 99.6%, 97.2%,95.2% and 70.7%, respectively (Dong et al., 2009). Acid-washed ZVI/ZVAl was used to remove Cr(VI), Zn and Ni in heavy metal waste-water with a removal rate of 99% (Han et al., 2016a,b). Pumice,perlite and lime were also used to remove Pbwith a removal rate of99% (Ranjbar et al., 2017).

2.2. Electrokinetic remediation

Electrokinetic (EK) remediation, a technology of running elec-trical current through pairs of electrodes across a contaminated soilbody to remove heavy metals and organic contaminants, hassteadily received growing attention in countries including UnitedStates and Korea since the mid-1990s (Fig. 3) (Acar andAlshawabkeh, 1996). Electrokinetic remediation is especially suit-able for soils of low permeability and strong adsorption capacity,i.e., clay or silt, where other remediating agents have difficultyreaching the interaggregate space.

The fundamental operating principles of electrokinetic reme-diation are summarized as follows. Under external electric current,the cathode produces hydroxide ions (4H2Oþ 4e� ¼ 4OH� þ

Table 7Removal rate of contaminant using EK.

Removal Electrode types Contaminant Condition of removal Type ofexperiment

Initialconcentrationof contaminant


Removaltime

Reference

EK COD The cathode: Al Theanode: Ti/RuO2-IrO3

Oxidation, precipitation

landfill leachate, Changsha,China

current density: 0.1 A/cm2, pH 6.37,Conductivity 12.05e13.08mS/cm, chlorideion concentration 6.5 g/L

lab 3640e4296mg/L

83.93 150min Li et al. (2016)

Ammonianitrogen

The cathode: Al Theanode: Ti/RuO2-IrO3

Oxidation, precipitation

landfill leachate, Changsha,China

current density: 0.1 A/cm2, pH 6.37,Conductivity 12.05e13.08mS/cm, chlorideion concentration 6.5 g/L

lab 1840e2042mg/L

100 150min Li et al. (2016)

Fe The cathode: AlOxidation, precipitationFe þ 2Hþ/ Fe2þ þH2

Pulau Burung Landfill,Malaysia

pH5; 2.5 V lab 96.81 60min Mahmad et al.(2015)

The cathode: stainlesssteelOxidation, precipitationFe þ 2Hþ/ Fe2þ þH2



Cr(Ⅵ) The cathode: AlOxidation, precipitationFe þ 2Hþ/ Fe2þ þH2


pH3; 2.5 V lab . 72.65 60min Mahmad et al.(2015)

The cathode: stainlesssteelOxidation, precipitationFe þ 2Hþ/ Fe2þ þH2



EK þ GAC Natural saline-sodic soil,spiked with contaminantmixture (kerosene, phenol,Cr, Cd, Cu, Zn, Pb, and Hg)

Hydraulic conductivity6.91� 10�9 cm/s,Electrical conductivity8.62 dS/m, voltagegradient 1 V/cm

lab 75.9 21d Lukman et al.(2013)

the synthetic AC-Feparticles are in soil Theanode: Graphite plateThe cathode: Stainless-steel plate

from industrial site 1 V/cm; Na2SO4 0.1mol/L, Conductivity 51.1mS/cm

lab 39.4mg/L 80.2 10d Yan et al.(2018)

The cathode:precipitation

leachate from Kahrizaklandfill, Tehran, Iran

current density 1A;initial pH¼ 7

batch 1.39± 0.42mg/L

84 60min Rabbani et al.(2012)

The anode/The cathode:Fe/Fe Pt Ti/Fe Al/Al Pt Ti/Al The cathode:Aluminum hydroxideprecipitation

Na2SO4 1 g/L and NaCl0.5 g/L

lab 1000mg/mL 100 60min Mouedhenet al. (2009)

Zn EK þ GAC Adsorption Natural saline-sodic soil,spiked with contaminantmixture (kerosene, phenol,Cr, Cd, Cu, Zn, Pb, and Hg)100mg/kg



The anode andtThecathode were reticularstainless steel

Artificial soil (60% kaolin,40%sand); Kaolin

Voltage gradient 1.5(v/cm), Brij35 1%, EDTA0.1M, Na2SO4 0.1M

lab 18mg/kg 63 15d Saberi et al.(2018)

Two square titaniumalloy electrodes

red soil Electrolyte refreshed ata rate of 40 rpm, thecatholyte pH wascontrolled by lactic acidand CaCl2

lab 265mg/kg 65 554 h Zhou et al.(2005)

Pb The anode and Thecathode were reticularstainless steel

Artificial soil (60% kaolin,40%sand); Kaolin

Voltage gradient 1.5(v/cm), Brij35 1%, EDTA0.1M, Na2SO4 0.1M


EK þ GAC Natural saline-sodic soil,spiked with contaminantmixture (kerosene, phenol,Cr, Cd, Cu, Zn, Pb, and Hg)100mg/kg

Hydraulic conductivity6.91� 10�9 cm/sElectrical conductivity8.62 dS/m voltagegradient 1 V/cm


EK kaolinite 133 mA/cm2 pilot scale 109.3mg/kg 0.55 2950 h Acar andAlshawabkeh(1996)

Anode (oxidation) Fe–> Fe2þþ 2e�

Cathode (reduction)2H2O þ 2e�

d>H2 þ 2OH�

landfill leachate samples(Thailand)

the electrical currentratio between reactingsurface areavolume of reactoroperation time

batch 30mg/L 99 Thaveemaitreeet al. (2003)






Removaltime

Reference

Fe þ 2H2O d>Fe(OH)2 þ H2

following a first-orderreaction.

Non-uniformElectrokineticpermeable reactivecomposite electrodeelectrokinetic (PRCE-EK)remediationThe anode:GraphiteelectrodeThe cathode:PRCE

soil voltage gradient 1.7 V/cm

lab 109.2mg/kg 69.3 480 h Liu (2013)

Cd EK þ GAC Adsorption Natural saline-sodic soil,spiked with contaminantmixture (kerosene, phenol,Cr, Cd, Cu, Zn, Pb, and Hg)

Hydraulic conductivity6.91� 10�9 cm/sElectrical conductivity8.62 dS/m voltagegradient 1 V/cm


Non-uniformelectrokineticpermeable reactivecomposite electrodeelectrokinetic (PRCE-EK)remediationThe anode: GraphiteelectrodeThe cathode: PRCE


lab 108.9mg/kg 89.9 480 h Liu (2013)

Ni anode and cathode werereticular stainless steel

Artificial soil (60% kaolin,40%sand);Kaolin

Voltage gradient 1.5(v/cm),Brij35 1%, EDTA 0.1M,Na2SO4 0.1M


Cu EK þ GAC Adsorption Natural saline-sodic soil,spiked with contaminantmixture (kerosene, phenol,Cr, Cd, Cu, Zn, Pb, and Hg)


lab 41 21d Lukman et al.(2013)

Two square titaniumalloy electrodes

red soil Electrolyte refreshed ata rate of 40 rpm, thecatholyte pH wascontrolled by lactic acidand CaCl2

lab 318mg/kg 63 554 h Zhou et al.(2005)

The cathode:Copperhydroxide precipitation

leachate from Kahrizaklandfill, Tehran, Iran

current density 1A,initial pH¼ 7

batch 2.28± 0.24mg/L

95 60min Rabbani et al.(2012)

EK þ PRB COD COD was oxidized nearthe anode

Laboratory preparedcontaminated soil

50ml 0.1mol/L KCl and10ml 0.05mol/L Citricacid solution,voltagegradient 0.35 V/cm

lab anode:71.5mg/Lcathode:71.9mg/L

anode:40.27cathode:90.40

Hu (2013)

Ammonia-N

ammonia is converted tonitrogen

current density 10mA/cm2, flow rate 6mL/min

lab 20mg/L 70 Mao et al.(2018)

Nitrate-N GAC PRBTwo graphite electrodeswere embedded in thesoil layer and under thereactive mediarespectively

synthetic contaminatedwater

carbon to sand ratios1:1, flow rate 2.3 L/min,pH¼ 6.8, voltage 30 V

lab 135mg/L 90 111 h Ghaeminia andMokhtarani(2018)

Pb the anode: graphiteelectrode þ zeolite PRLthe cathode: iron PRL,zeolite PRL þ graphiteelectrode


lab 371.0mg/kg 47.1 15d Fu et al. (2012)

Pb was precipitated nearthe cathode andadsorbed by PRB

Laboratory preparedcontaminated soil

50ml 0.1mol/L KCl and10ml 0.05mol/L Citricacid solution,voltagegradient 0.35 V/cm

lab anode:91.8mg/Lanthode:93.2mg/L

anode:57.84cathode:92.6

Hu (2013)

Cd the anode: graphiteelectrode þ zeolite PRLthe cathode: iron PRL,zeolite PRL þ graphiteelectrode


lab 500.1mg/kg 49.4 15d Fu et al. (2012)

Ni the anode: graphiteelectrode þ zeolite PRLthe cathode: iron PRL,zeolite PRL þ graphiteelectrode


lab 395.7mg/kg 39.2 15d Fu et al. (2012)

Cu the anode: graphiteelectrode þ zeolite PRL


lab 464.7mg/kg 36.7 15d Fu et al. (2012)

(continued on next page)






Removaltime

Reference

the cathode: iron PRL,zeolite PRL þ graphiteelectrode

EK þ BIO Pb Cathode: reductionprecipitationAnode: Graphite FiberElectrodeBiosorption

Industrial waste anode:83 cathode:62

Colantonio andKim (2016)

Zn Bioleaching,electrokinetic (EK) andBEER technology

contaminated sludge acidophilic bacteria;inoculum dosage of theacclimatized bacterialstrain 4%, pH5

lab 1000 mg/kg 72.86(BIO),56.67 (EK),93.08(BBER)

96 h96 h72 h

Adikesavanand Rajasekar(2018)


2H2[), which then form hydroxide precipitates with heavy metalions; the anode produces oxygen and free radicals (4OH� � 4e� ¼2H2Oþ O2[), which in theory could oxidize COD and nitrate-nitrogen due to the electrochemically produced stronglyoxidizing hydroxyl radicals (Zhang et al., 2001; Virkutyte et al.,2002; Qiao et al., 2004).

However, limited studies have been reported on EK removal ofCOD and ammonia nitrogen. Li et al. (2016) reported that theremoval efficiencies of the EK method for COD and ammonia ni-trogen from a landfill leachate are 83.7% and 100%, respectively.

More complicated mechanisms were reported: (1) Al3þ ions werereleased from aluminum cathode, dissolved in the solution, andproduced sludges, which had a coagulation effect on COD andammonia nitrogen, and (2) On Ti=RuO2 � IrO2 anode, there aredirect oxidation (by transferring electrons directly, 4H2Oþ 4e� ¼4OH� þ 2H2[) and indirect oxidation, both of which can removeCOD and ammonia nitrogen.

The removal efficiency by the EK method depends on manyfactors, including electrode material, pH, voltage gradient, andcurrent density. Anode materials are usually inert to chemical re-actions and high electrical conductivity, including titanium,graphite, platinum, or sometimes sacrifice electrode such asaluminum; while cathode materials are usually corrosive-resistantin basic environments, including aluminum and iron (Alshawabkehet al., 1999). Mahmad et al. (2015) reported that the highest ironremoval efficiencies are 96.81% (pH 5, voltage 2.5 V, aluminumelectrodes) and 94.3% (pH 4, voltage 2.5 V, stainless steel elec-trodes), respectively. The dissolution of the stainless-steel electrodewas postulated to attribute to the slightly lower removal rate. Kimet al. (2011) reported EK-solar cell remediation for a soil contami-nated with Cd, Cu, and Pb, and found that the maximal removalefficiencies of Pb and Cd were achieved at 90% with addition of50mM of citric acid.

Many innovative techniques were developed over the past de-cades to improve the removal efficiency of EK. Placing iron-loadedactivated carbon in Cr(VI)-contaminated soils improved removalefficiency due to adsorption of Cr(VI) on AC and reduction of Cr(VI)

by Fe2þ (Cr6þ þ 3Fe2þ þ AC/Cr3þ þ 3Fe3þ þ AC). The highestremoval efficiency (80.2%) was achieved at an AC-Fe ratio of 5%,with graphite and stainless-steel plates as the anode and cathode,respectively (Yan et al., 2018). A hexagonal unit cell, with six anodesat corners and one cathode at the center, was constructed to in-crease the hydroxyl production at the cathode. This enhanced for-mation of metal hydroxide to remove heavy metals (Liu, 2013). Toprevent the precipitates from clogging the pores near the cathodeand from depositing directly onto the cathode, the cathode was

replaced every 120 h. Average removal efficiencies of Cd (89.9%)and Pb (69.3%) were achieved after 480 h (Liu, 2013) (Table 7).

Colantonio and Kim (2016) reported that the removal rate of Pb2þ is83% in the anode chamber, open circuit and 62% in the cathodechamber, open circuit, using lab-scale microbial electrolysis cells(MECs) with an anion exchange membrane (AEM) (Colantonio andKim, 2016). Furthermore, alternating electrical field was used toneutralize pH near electrodes, thereby improving the removal ef-ficiencies of metal ions.

The electrokinetic (EK) technique was also used in conjunctionwith other remediation techniques, such as permeable reactivebarrier and microbes to further increase efficiency and tackle soiland groundwater with complex contaminants. Wastewater con-

taining both COD and Pb2þ was treated with both EK and PRB (Hu,2013). During the migration of contaminants from anode to cath-ode under an external electric field, COD was oxidized near theanode, while Pb was precipitated near the cathode and was thenadsorbed by PRB. The removal efficiencies of COD and Pb were57.03% and 81.62% in the anode zone, and were 90.4% and 94.6% inthe cathode zone, respectively (Hu, 2013). The molecular sieve,which is commonly composed of zeolite, was installed adjacent tothe cathode and can prolong the life of cathode and adsorb heavymetals. Meanwhile a ZVI layer installed near the anode can reduceheavy metals. The combination of the zeolite molecular sieve nearthe cathode and the ZVI layer near the anode yielded removal ef-ficiencies for lead, cadmium, nickel and copper of 49.4%, 47.1%,39.2% and 36.7%, respectively (Fu et al., 2012). Adikesavan andRajasekar (2018) reported that bioleaching enhanced electroki-netic remediation (BEER) technology improved the removal effi-ciency of zinc (93.08%), where the anode optimized the pH toproduce acidophilic bacteria, and the acidic environment createdby acidophilic bacteria increased the solubility of heavy metalsattached to the soil, thereby promoting its electromigration.

2.3. Microbial remediation

Microbial remediation of contaminated sites is economical,sustainable, and has a shorter treatment cycle than phytor-emediation. Previous studies focus on the remediation of a singletype of heavy metals or xenobiotic organic compounds, or a com-bination of the two (Liu et al., 2015a,b; Jiang et al., 2015; Chen et al.,2015; Mandal et al., 2016; Gong et al., 2018). Few studies have beenperformed on microbial remediation of contaminated soils orleachate in municipal solid waste landfill sites, which containcomplex components (Table 8).

Bacteria, fungi, and actinomycetes are commonly used for


remediation of contaminated sites (Liu et al., 2015a,b; Pinedo-Rivilla et al., 2009), and have been known to achieve a highdegradation efficiency of COD by intracellular/extracellular en-zymes and co-metabolism of microorganisms. Under the optimalgrowth conditions (pH¼ 7, inoculum size 3%, 25 �C), the cold-tolerant strain, isolated and screened from the activated sludge ofthe sewage treatment plant, catalyzed a series of biological redoxreactions using the dehydrogenase produced by the strain, andremoved 78.8% COD (initial concentration of 800mg/L) after 96 h(Gao et al., 2015).

Microbial remediation techniques for heavy metal removal arerelatively mature with high removal rates, including bio-accumulation (precipitating or chelating heavy metal ions on bio-polysaccharides, binding contaminants to the extracellular matrix,or electrostatic adsorption), and biotransformation (methylation,demethylation, or redox), with which heavy metals are eitherpassivated or eliminated (Liu et al., 2015a,b; Ye et al., 2017). Akaret al. (2007) utilized biomass, which produced by Aspergillus par-asiticus fungi, to remove Pb(II) from wastewater through ion ex-change (mainly) and the organic complexation process. At pH¼ 5,20 �C, and an initial concentration of 100mg/dm3 of Pb(II), aremoval rate of APG-Biomass (1.6 g/dm3) was reached at83.98± 2.22% after 70min. The multi-metal resistant endophyticbacteria L14 (EB L14) increased the removal efficiency of heavymetals in wastewater by inhibiting the ATPase (Guo et al., 2010).After 24 h, the removal rates of Cd(II) and Pb(II) at initial concen-tration of 10mg/L reached 75.78% and 80.48% (Guo et al., 2010).Contaminated soils can also be remediated by microorganisms. Cuiet al. (2017) employed a composite microbial agent to repair tail-ings contaminated soil. After 49 days, 74.98% of the total Zn(C0¼ 73.2mg/kg), 85.29% of total Pb (C0¼13.55mg/kg), and 79.41%of total Mn (C0¼1070mg/kg) inside the soil were stabilized. P.ostreatus (macro fungi) took straw as a nutrient and adsorbed81.25% and 68.86% of Pb and Ni from a contaminated soil takenfrom anMSW landfill site to its fruiting body after 22 days (Bharathet al., 2019).

Even though the basic elements for microorganism growth cantheoretically be consumed via metabolism, nitrogen and phos-phorus have rarely been reported as removed microbially fromlandfills until recently. With 30 g/L of bamboo charcoal-probioticsEM.1 (3%), 89.5% of ammonia-nitrogen with an initial concentra-tion of 387mg/L, and 68.29% of total phosphorus with an initialconcentration of 20.16mg/L were removed after 48 h (Jiang et al.,2012).

Bioremediation efficiency is restricted by the following factors:(1) a microorganism usually can handle only one group of con-taminants; (2) the removal efficiency is sensitive to pH and tem-perature. If pH or the temperature is away from optimal conditions,enzyme activity will be reduced, the spatial structure of microor-ganisms will be destroyed (Gao et al., 2015), and binding sites andsurface charge will be dissociated (Akar et al., 2007), which resultsin lower removal ability; and (3) sometimes themicrobial activity isinhibited by high concentrations of pollution sources.

In order to enable bioremediation technology to remove mul-tiple contaminants simultaneously and improve its efficiency, bio-enhancing agents, which increase the biodiversity, supply elec-tron acceptors, and provide sufficient oxygen and nutrient salts(such as carbon-nitrogen and carbon-phosphorous at ratios of 10:1and 30:1 in Kensa (2011)) were suggested.

2.4. In-situ injection treatment

2.4.1. Nano zero-valence ironNano zero-valence iron (nZVI), due to its large specific surface

area and strong reducing ability, has been widely used in

remediation of sites contaminated by chlorinated organic matters,heavy metals and nitrate. Nano-sized ZVI particles are mixed in aslurry, injected through the contaminated area by either naturalgroundwater flow or pumped to react with the target contaminants(Wang et al., 2018; Wei et al., 2010). Given nZVI particles are proneto agglomerate during migration, which leads to a reduction inspecific surface areas, a reduced migration distance, a deteriorationof reactivity, and reduced efficiency. To solve these problems, thesurface of nZVI is often passivated, loaded, dispersant/stabilizermodified, or coated to form bimetallic composite (Gao and zhou,2013).

The efficiency of nZVI to remove a variety of heavy metals, suchas copper (II), chromium (VI), mercury (II), and lead (II), is oftenvery high (>90%, Table 9) (Xiao et al., 2011; Wang et al., 2018; Xionget al., 2009; Zhang et al., 2010). Multiwalled carbon nanotubesloaded with nZVI (MWCNT-nZVI) could convert copper (II) to an Fe/Cu alloy and deposit it on its surface. At pH¼ 4.5, the concentrationof copper (II) is 50mg/L; the concentration of MWCNT-nZVI is0.5mg/mL, and the removal rate reaches 94% after a reaction timeof 60min (Xiao et al., 2011). Carboxymethyl cellulose with ironsulfide (CMC-FeS) or with nano zero-valence iron (CMC-nZVI), astabilizer modified nZVI, can reduce contaminants dissociated frompolluted soils into groundwater (Xiong et al., 2009; Wang et al.,2018). When the molar mass ratio of CMC (0.2%) FeS to mercury(II) in the soil is 26.5, the removal rate after 7 days reached 97%(leaching to water) and 99% (leaching to TCLP fluid) (Xiong et al.,2009); whereas the leaching removal rate, quantified in theleaching batch experiment (Table 9) is the ratio of the difference ofleachate concentrationwith and without adding nZVI to soil (initialconcentration) to initial concentration. Wang et al. (2018) injected1.5 kg of CMC-nZVI into the injection well, which reduced thechromium (VI) concentration from 2mg/L to less than 0.1mg/L in a6m� 6m contaminated site. Pilot scale tests of nZVI injection inporous aquifer and fractured bedrock for the removal of Cr(VI),chlorinated aliphatic hydrocarbons, Ni, Pb and nitrate were alsoreported (Mueller et al., 2012; Müller and Nowack, 2010; Bardoset al., 2014) (see Table 10).

nZVI can convert nitrate into ammonium, which leads to highremoval rate of nitrate-nitrogen but a low removal rate of totalnitrogen (Tang et al., 2012). If nZVI is coupled with denitrifyingbacteria, most of the final products will be gaseous nitrogen, whichreduces the formation of ammonia nitrogen and nitrite and in-creases the removal rate of total nitrogen (Wang et al., 2015). Wanget al. (2015) reported that 2 g sodium oleate-coated nZVI and 10mLof activated bacterial solution removed 80.3% nitrate (155mg) after6 days and 80% of the final product was gaseous nitrogen.

Although nZVI has been extensively adapted for the remediationof contaminated groundwater, many problems remain unsolved:(1) High cost: The Pd-nZVI (bimetallic composite nZVI) is 50 USdollars (Wei et al., 2010) per kilogram. (2) In the presence of oxygen,it is easily oxidized to iron (II) or iron (III). Moreover, the dissolvedoxygen or other oxides can easily cause nZVI surface passivation,which interrupts the reaction. (3) In situ injection of nZVI is oftenaccompanied by its agglomeration and sedimentation, resulting ina shortermigration distance and smaller remediation range. (4) Thetransport process of nZVI in the soil is likely a preferential flow (Shiet al., 2015), so that the contaminated site cannot be uniformlyremediated (only the contaminated area near the preferential flowis handled); thus, the nZVI on the end of the preferential flow willaccumulate, which adds a secondary pollution to the environment.

2.4.2. Solubilizing agentSolubilizing agents, containing both hydrophobic groups and

hydrophilic groups, are water-soluble chemical compounds thatcan be used to dissolve contaminants adsorbed on the soil. The

Table 8Studies of the treatment of contaminants detected in MSW leachate by microorganisms.

Description of contaminants Description of the remover/removal method Description of the removal effect References

Type ofpollutant

Contaminant Initialconcentration

Remover Dosage ofremover

Condition ofremoval

Removal mechanism Type ofexperiment

Removaltime

Removalrate/removalcapacity

COD Simulateddomesticsewage

800mg/L cold-tolerantstrain

3%Inoculumdose(volumefraction)

pH¼ 7.0, 25 �C biological redox reactionscatalyzed by microorganismproduced DHAase

batch 96 h 78.8% (71.2%,4 �C)

Gao et al.(2015)

Simulatedcoal minewastewater

1372 -1680mg/L

sulfate-reducingbacteria(SRB)-maifanStone

100mlSRB,475 cm3

maifanStone

COD/SO4

2�¼ 2.0,adequatecarbon supply,hydraulicload¼ 0.1m3/(m2$d)

Maifan stone has goodadsorption capacity,dissolution properties andporous structure, and servesas immobilization carrier ofmicroorganism

column 54d 46.70% Di et al.(2014)

Biogas slurry 4494.1mg/L probioticsEM.1

bamboocharcoal(30 g/L)-EM.1 (3%)

pH¼ 7.0e8.0,DO¼ 2mg/L

Bamboo charcoal serves as amicrobial immobilizationcarrier; synergistic effect ofbamboo charcoal adsorptionand microbial degradation.

batch 48 h 66.30% Jiang et al.(2012)

Inorganic NH3 Biogas slurry 1167.25mg/L probioticsEM.1


pH¼ 7.0e8.0,DO¼ 2mg/L



Leachatepretreated byA/O process

387mg/L BWFmicrobialagent

sample/solutionratio¼ 8 g/10 L

e e batch 7d 89.50% Liu andZhang(2016)

TP Biogas slurry 20.16mg/L probioticsEM.1


pH¼ 7.0e8.0,DO¼ 2mg/L



Heavymetal

Ni Soilcontaminatedwith MSW

e Pleurotusostreatus(mushroomspecies)

e straw servedas nutrientsource

Heavy metals wereaccumulated onto thefruiting bodies ofmushrooms

batch 22d 68.86% Bharathet al.(2019)

Pb 81.25%

Zn(II) Heavy metalstock solution

20mg/L compositemicrobialagent

1.2ml pH¼ 7, 26 �C Monolayer-based ionexchange and complexation

batch 24 h 86.89% Cui et al.(2017)Pb(II) 1.0ml pH¼ 6, 26 �C 95.46%

Mn(II) 1.0ml pH¼ 6, 28 �C 92.78%Zn Mine tailings

soil73.2mg/kg 8e12 g/m2 e (1) Certain specific enzymes

or extracellular polymers,which can react with heavymetals, were secreted by themicrobes; (2) Heavy metalswere combined with organicacids (produced in metabolicprocess)

49d 74.98%Pb 13.55mg/kg 85.29%Mn 1070 mg/kg 79.41%

Pb citric acidleached liquidfromcontaminatesoil

e Klebsiellaoxytocastrain BAS-10isolated fromsedimentsfrom pyritemine tailings

e Anoxicenvironment,28 �C

An iron gel (produced byBAS-10 using residualcitrate) that co-precipitatesheavy metals

batch 6d 100.00% Baldi et al.(2007)Cr 92.00%

Fe 86.00%Cd 93.00%Zn 80.00%Cu 79.00%

Cd(II) Heavy metalstock solution

10mg/L multi-metalresistantendophyticbacteria L14(EB L14)

e 30 �C restraining the activities ofATPase

batch 24 h 75.78% Guo et al.(2010)Pb(II) 10mg/L 80.48%

Cu Metal stocksolution

10-2M extracellularpolymericsubstances(EPS)

0.45mg pH¼ 7e8,25± 1 �C,under nitrogenatmosphere,KNO3 servedas asupportingelectrolyte

e batch e 5304 mmol/g Comteet al.(2008)

Pb 10�2M 2509 mmol/gCd 10�3M 85 mmol/g

Pb(II) Metal stocksolution

100mg/dm3 Aspergillusparasiticusfungalbiomass(APF)

1.6 g/dm3 pH¼ 5, 20 �C Ion exchange (primarily);complexation of lead ionswith APF functional groups

batch 70min 83.98± 2.22% Akar et al.(2007)


Table 9Studies of the treatment of contaminants detected in MSW leachate by nZVI.

Description of contaminants Description of the remover/removal method Description of the removal effect References

Type ofpollutant

Contaminant Initialconcentration

Remover Dosage of remover Condition of removal Removal mechanism Type ofexperiment

Removaltime

Removalrate

Limitation

Nitrate NaNO3

simulatedwater sample

70mgN/L sodium oleatecoated nZVI-denitrifyingbacteria

2 g sodium oleate coatednZVI, 10mL activatedbacteria solution, 500mLNaNO3 simulated watersample

15 �C, dark and anaerobicenvironment, dissolvedoxygen¼ 0.54mg/L

Nano-iron reduces nitrate toammonia nitrogen; thesodium oleate severed as acarbon source for denitrifyingbacteria

batch 6d 80.30% e Wanget al.(2015)

NaNO3

solution60mgN/L ZVI-raw loess

soilratio of liquid to soil V/W¼ 5:1

1.0mM Fe3þ in NaNO3

solutionNitrate-N converted toammonium-N by ZVI

batch 30 h 100.0% The removal rate ofnitrate is high, but thetotal nitrogen removalrate is low

Tang et al.(2012)

Heavymetal

Cu(II)

CuSO4

solution50mg Cu/L MWCNT-nZVI [mat]¼ 0.5mg/mL pH¼ 4.5 chemical reduction batch 60min 94.0% Need more studies on

using MWCNT-nZVI toremove other heavymetals, such as Pb(II),Cr(VI), and As (V)

Xiao et al.(2011)

TotalCr

Cr(VI) loadedsandy loamsoil

83mg/kg CMC-nZVI sample/soilratio¼ 0.0012 g: 1.5 g

pH¼ 9.0 Degradation of chromium(VI) to trivalent chromium

leachingbatch

24 h 50%(leachinto DIwater)

e Xu andZhao(2007)

Cr(VI)

34mg/L CCMC-nZVI¼ 0.12 g/L e batch 48 h 90.0%

Hg(II)

Mercuryspiked clayloamsediment

177mg/kg CMC(0.2%)-FeS molar ratio of FeS-to-Hg:26.5

22± 0.1 �C, pH¼ 7.0± 0.1 Adsorption and co-precipitation, convertingHg(II) to HgS

leachingbatch

7d 97%(leachinto DIwater);99%(leachinto TCLP)

Further research inneed on how to reduceHg methylation,decrease costs andimproveenvironmentalfriendliness.

Xionget al.(2009)

Pb(II)

electroplatingwastewater

12.75mg/L K-nZVI(20%nZVI)

sample/solutionratio¼ 0.1 g/10ml

30 �C K-nZVI has large specificsurface area andsubsequently betteradsorption and reductionproperties; kaolin preventsagglomeration of iron

batch 60min 98.8% e Zhanget al.(2010)

J.Yeet

al./Chem

osphere227

(2019)681

e702

695

Table 10Studies of the treatment of contaminants detected in MSW leachate by solubilizing agent.

Description of contaminants Description of the remover/removal method Description of the removaleffect

References

Type Contaminant Initialconcentration

Remover Dosage of remover Condition Removal mechanism experiment time Removalrate

Cu clean soils mixed withPb(NO3)2 and CuCl2solution

122.3mg/kg EDDS-saponinbiosurfactant

(0.08 mmolEDDS þ 24 mgsaponin): 1 g soil

25 �C complexation of thesaponin micelles andmetals; enhanced byEDDS

batch 48 h 85.7% Cao et al.(2013)Pb 184.4mg/kg 99.8%

Zn Soils heavilycontaminated (morethan 80 years) withsulfuric acid and metalsdeposited in the air

325mg/kg EDTA(0.1M)-Ammonyx KP (nonionicsurfactant)

sample/soilratio¼ 0.02ml: 10 g

25± 1 �C,pH¼ 6.2

EDTA served as achelating agent

batch 48 h 99.2% Slizovskiyet al.(2011)

Cu 1565 mg/kg 100.0%Pb 3245 mg/kg 98.7%Cd 59mg/kg 99.2%Zn 325mg/kg I (0.15M)- Ammonyx

KP (nonionicsurfactant)

Stable metalliccomplexes wereformed with iodide andsome heavy metals

52.6%Cu 1565 mg/kg 71.1%Pb 3245 mg/kg 76.8%Cd 59mg/kg 67.3%Ni contaminated soil from

an industrial park151.00mg/kg saponin biosurfactant sample/soil

ratio¼ 0.00375: 1 g26e29 �C,pH¼ 5

e batch 72 h 99.0% Maity et al.(2013)Cr 70.00mg/kg 73.0%


dissolved contaminants were further collected by seepage inducedby either natural hydraulic gradient or by pumping (Strbak, 2000;Paria, 2008; Lee et al., 2007; Mao et al., 2015) during remedia-tion. Common solubilizing agents are surfactant and cosolvent.

Due to hydrophobic affinity, solubilizing agents have beenwidely used to remediate non-polar contaminants, e.g., dense non-aqueous phase liquids (DNAPLs), benzene, toluene, ethylbenzeneand xylenes (BTEX) from gasoline spills, and oil frompetroleum andchemical industries. The solubilizing agents have achieved goodtreating effects. Research on this aspect was relatively mature,including lab-scale (Ma et al., 2016; Sun et al., 2013; Taylor et al.,2004; Maire and Fatin-Rouge., 2017), pilot-scale (Tick et al., 2015)and full-scale (Strbak, 2000; Lee et al., 2014) experiments.

A p, p’-DDT contaminated soil column with an initial concen-tration of 990mg/kg was flushed (flow rate¼ 6ml/h) with cosol-vent 1-propanol (40%), which reduces the water-p, p’-DDT surfacetension andmakes the contaminantmiscible with water. After 50 h,96% p, p’-DDT was desorbed (Juhasz et al., 2003). Sun et al. (2013)utilized 100 g/L methyl b-cyclodextrin (a biosurfactant) in combi-nation with a 50 �C water bath and 30min of 35 kHz ultrasonicfortification to remove PAHs from the soil. After 60min, 91.3± 3.1%of total PAHs (initial concentration¼ 337.34± 9.54mg/kg) wereremoved. Maire and Fatin-Rouge (2017) showed that only two porevolumes were needed to reach a DNAPL removal efficiency of 95%when the soil column was preflushed by a synthesized surfactantfoam with nitrogen and a dihexyl sulphosuccinate (DHSS) surfac-tant, and then treated with Tergitol 15S9 (a surfactant). As a com-parison, 40 pore volumes were needed to reach a DNAPL removalefficiency of 90% when only treated with Tergitol 15S9. However,few scholars reported treating dissolved organic matter (quantifiedas COD) in MSW landfills by a solubilizing agent.

Surfactants and cheating agents can also remove heavy metalsby complexation and ion exchange, which can be enhanced byacidic solutions. At the sample/soil ratio of 0.00375 g: 1 g, thesaponin biosurfactant removed 99% of Ni and 73% of Cr, after 72 hbased on initial concentrations of 151.00± 1.98mg/kg and70.00± 1.12mg/kg, respectively (Maity et al., 2013). After treatingheavy metal-contaminated soil with ethylenediaminetetraacetateacid (EDTA) (0.15M)-Ammonyx KP (nonionic surfactant) for 48 h,removal rates of Zn, Cu, Pb, and Cd (initial concentrations wererespectively 325mg/kg, 1565mg/kg, 3245mg/kg, 59mg/kg)reached 99.2%, 100%, 98.7%, and 99.2%. (Slizovskiy et al., 2011).

The solubilizing agent is relatively inexpensive, as the synthetic

surfactant cost is 1e2 V/kg. However, these issues are noted:(1)Solubilizing agents occasionally adsorbed on the surface of the soilparticles due to the strong affinity of the hydrophilic group to soilparticles, resulting in reduced efficiency and a second pollution(Mao et al., 2015). (2) After the surfactant is reused three times, itssolubilization effect is not obvious (Sun et al., 2013).

2.4.3. Micro and nanobubblesMicro and nanobubbles (MNBs) are tiny bubbles with a radius in

the range of 0.05 mme25 mm and can be produced by decompres-sion method, gas-water circulation method, electrolysis, gas flota-tion pump method, micro-pipeline, high-temperature technology,ultrasonic technology (Agarwal et al., 2011; Deng et al., 2014; Wanget al., 2017), or a combination of above-mentioned methods (Liet al., 2013; Agarwal et al., 2011). The mechanisms of contami-nants removal by MNBs are as follows. (1) Due to their high surfacecharge, large specific surface area and long life in water, MNBsadhere to flocs and suspended matter in the pore fluid efficiently(Wang et al., 2017). Under buoyancy or seepage effects, bubbles andthe adhered contaminants gradually migrate upward to the watersurface (2) The high gas-liquid mass transfer rate of MNBs makesthe dissolved oxygen supersaturated in water, thereby promotingthe degradation of pollutants by aerobic microorganisms. (3) Due tothe high charge density of the electric double layer of MNBs nearthe water-air interface, at the moment the MNB collapses, the en-ergy accumulated from the densely-packed positive and negativeions is released and excites the generation of many hydroxyl radi-cals with a strong oxidizing ability (Xiong et al., 2016). (4) Due tothe strong oxidizing ability of the ozone, a variety of organic mat-ters, which are hard to be decomposed otherwise, can be oxidizedby ozone MNBs.

MNBs have been widely used in industrial wastewater treat-ment and in situ remediation of contaminated groundwater. Afterraw dyeing wastewater was treated by the coagulation (coagulantdosages¼ 100mg/L)-air micro bubble flotation processes for 5min,the removal rate of COD (initial concentration¼ 5051.1mg/L)reached 89% (Liu et al., 2010). The coagulation (coagulant dos-ages¼ 100mg/L)-ozone microbubble flotation process achieved a64% removal rate of ammonia from raw coke wastewater after2min flotation time (Liu et al., 2012). Groundwater containingmultiple persistent organic contaminants was treated by ozonemicrobubbles, with a removal rate of over 95% of benzene (initialconcentration¼ 30.5± 5.1mg/L) and chlorobenzene (initial


concentration¼ 14.8± 2.5mg/L), and 67% of nitrobenzene (initialconcentration¼ 502.8± 57.5mg/L) after 30min (Xia and Hu, 2019).

Ozone bubbles have been used to conduct in situ remediation ofgroundwater contaminated by methyl orange using column scaletests with removal rate of 98% (Hu and Xia, 2018). Ozone bubbleshave also been used for field scale tests on trichloroethylene (TCE)with removal rate of 99% (Hu and Xia, 2018) and on methyl-t-butylether (MTBE) with removal rate of 80% (Kerfoot, 2002), and ondiesel (Kerfoot, 2003). Oxygen bubbles can promote in situbiodegradation efficacy by supplying the electron acceptor andhave been used for the removal of p-xylene (Jenkins et al., 1993)and phenanthrene (removal rate¼ 51.5%, Choi et al., 2009) undercolumn scale tests. Hu et al. (2015) developed a process for the insitu remediation of contaminated sites by MNBs: MNBs with watercontaining nutrients were injected upstream of the contaminatedarea, which decomposed the pollutants and supplied the electronacceptors/donors for themicroorganisms in the contaminated area;Monitoring wells enabled real-time monitoring of organicpollutant removal processes; Pumping wells accomplished thesecondary utilization of MNBs water and formation of thegroundwater flow field. Meegoda and Batagoda (2016, 2017)developed a device consisting of an ozone-nanobubbles watersupport system and ultrasound probes for decontaminating poly-cyclic aromatic hydrocarbons (PAHs) from river sediments, with aremoval rate that reached 93% under optimal conditions withreasonable time intervals between ultrasound applications.

There are several shortcomings to using MNBs for in situremediation of contaminated sites: (1) Due to the manufacturingdifficulty, energy consumption and maintenance costs of MNBgenerating device, MNB technology is significantly more expensivethan traditional nitriding and ozonation methods in wastewatertreatment. (2) MNBs oxidize heavy metals, thereby improving theirtoxicity and mobility in groundwater, which increases the risk ofexposure to the public. For example, ozone micro and nanobubbleswould oxidize Cr(III) to Cr(VI) (Meegoda and Batagoda, 2016). (3)Most bubbles are generated on microscale (micro bubbles), whilegeneration of nanoscale bubbles is still difficult.

Currently, no reports are available on the treatment of landfillleachate or in situ remediation of landfills contaminated sites byMNBs technology. However, it is expected to be applied in this typeof treatment based on its successful application to the treatment ofCOD and ammonia in industrial wastewater and in situ remediationof the petroleum/chemical industry contaminated sites. Never-theless, more research is warranted.

2.5. Phytoremediation

Phytoremediation, a means of remediation and rehabilitationfor a contaminated land using green plants, has emerged as a viablealternative site remediation method for MSW landfills and dump-sites due to its large treatment area and capability to treat con-taminants distributed spatiotemporal variably throughout theshallow soil layers (Nagendran et al., 2006; Pathak et al., 2012;Lamb et al., 2014; Reddy et al., 2017). Mechanisms of phytor-emediation include (Pathak et al., 2012; Jones et al., 2006;Nagendran et al., 2006): (1) phytoextraction, where plants (e.g.,Thalaspi, Alyssum and Brassica) take up contaminants from soil orwater, then translocate and accumulate them into roots orharvestable shoots; (2) phytofiltration, where plants absorb con-taminants from contaminated surfacewater; (3) phytostabilization,where contaminants (metal and organic) are immobilized in thesoil by means of sorption (binding), precipitation, and complexa-tion by the deep or fibrous root systems of the plants; (4) phyto-volatilization, where contaminants are converted to volatile formand released into the atmosphere via evapotranspiration process of

the plants; (5) phytodegradation, where organic contaminants areconverted to less harmful substances by the plants; and (6) Rhi-zofiltration, where plant roots growing in polluted water precipi-tate and concentrate metals (Cu, Hg, Pb, Zn).

Major categories of contaminants from a solid waste landfill(Kjeldsen et al., 2002; Han et al., 2016a,b) defined in Introductioncan all be removed by phytoremediation. COD and NHþ

4 in landfillleachates can be efficiently reduced by either forestry or grasslandtreatment system (Jones et al., 2006). COD, nitrogen and phos-phorous in an anaerobic MSW landfill in the north of Italy wereremediated by sunflower with removing efficiencies of 50%, about100% and 100%, respectively (Garbo et al., 2017). Heavymetals fromKuchy�nky landfill leachate were removed by mushroom species (P.ostreatus as a bioaccumulator) (Vaverkov�a et al., 2017). Sitescontaminated with both organic and heavy metals were alsoeffectively treated with native species such as Switchgrass andLittle Bluestem (Reddy et al., 2017).

The major drawback of phytoremediation is the long treatmentcycle due to the relative slow growth rate of natural plants, whichwas also subject to the influence of climate and hydrologicalconditions.

3. Part 2. design chart for sustainable remediation (casestudies)

To account for the different features of hydrogeological char-acteristics of contaminated sites, a design chart was compiled forquick adaptation of the suitable technique for remediation of sitescontaminated by solid waste landfills (Fig. 4). Three cases weregiven as follows to show the design procedures.

3.1. PRB removing COD,Fe2þ and Cu2þ in an MSW landfill site

Chathuranga et al. (2016) reported a field-scale permeablereactive barrier (PRB) set-up, located in the proposed landfill-site inthe Hambantota Municipal Council, where the ground water was

contaminated by landfill leachate consisting of COD, nitrogen, Fe2þ,and Cu2þ. Flow rate across the PRB was 100mL/min (Chathurangaet al. (2016)). According to the proposed design chart (Fig. 4), PRB isa reasonable candidate for remediation. Indeed, a multi-layer PRBwas selected, with low-cost and locally available reactive materials,including firewood, charcoal, bio char, saw dust, washed quarrydust, dewatered alum sludge, red soil and washed silica sand. The

removal efficiencies of COD, Fe2þ, Cu2þ, and TN (total nitrogen)were 45%, 31%, 53%, and 49%, respectively, primarily throughadsorption, ion-exchange reactions, filtration and precipitationmechanisms in the sequential PRB (Chathuranga et al. (2016)).

3.2. PRB removing Fe2þ and Mn2þ

Wang et al. (2016) reported that permeable reactive barrier testswere carried out in the Klondike Landfill, a closed unlined munic-ipal solid waste landfill, located in Florida, USA. The site was

contaminatedwith Fe2þ andMn2þ, with a hydraulic conductivity ofapproximately 7.93m/day and a hydraulic gradient of 0.006. Thissite is most suitable for the PRB method, which is also suggested inthe proposed design chart (Fig. 4). PRB was divided into two equallength sections, filled with different reactive materials for Fe andMn removal. One reactive material was limestone, achieving anaverage removal rate of 91% for the first year of operation. Anotherreactive material was crushed concrete, achieving an averageremoval rate of 95% for the first year of operation. The removal ratesfor both reactive materials in the third year of operation declined toan average of 64% and 61%, respectively (Wang et al., 2016).

Fig. 4. Removal rate of suggested remediation technologies for (a) COD, (b) inorganic contaminants, (c) Fe, Mn, Cr(VI), Cd and (d) Zn, Pb, Cu, Ni.


3.3. EK removing Pb2þ

Acar and Alshawabkeh (1996) reported the removal efficiency oflead under electrokinetic remediation at pilot scale in kaolinitespiked with lead nitrate solution. The site was contaminated with

Pb2þ, with a hydraulic conductivity of 5� 8� 10�8cm=s and hy-draulic gradient of 10e20. This site is most suitable for the EKmethod, also given as an option in the proposed design chart (Ta-ble 11). A row of five equally spaced graphite rods that are chemi-cally inert to electrolysis reactions were installed as both the anodeand cathode. When a constant direct current density of 133 mA/cm2

is applied, Pb (lead) was transported toward the cathode and pro-duced hydroxide precipitation, with a removal rate of 55% (Acar andAlshawabkeh, 1996).

3.4. EK þ PRB removing Cu2þ

Chung (2009) reported an in-situ electrokinetic reactive piletests (electrokinetic remediation coupled with a permeable reac-tive barrier) adjacent to a waste landfill site in Incheon, Republic of

Korea, which was contaminated with Cu2þ, with a permeability of8:3� 10�4cm=s (iron powder), 1:2� 10�4cm=s (zeolite),2:4� 10�1cm=s (slag), 3:3� 10�2cm=s (tire chip), and1:8� 10�3cm=s (sand). This site is most suitable for EK and PRBmethods, as suggested in the proposed design chart (Fig. 4). EK-PRBwas chosen as the remediation method. Stainless rods wereinstalled inside both the anode and cathode hollow piles (made of60mm diameter PVC pipe, installed vertically to a depth of 2.5mbelow the ground surface). The spaces between the rod and pipewere filled with sand (the anode) and reactive materials (thecathode) respectively. Copper migrated from the anode side to-wards cathode, where it was adsorbed. The removal rates of Cuwere 68e68.7, 93.4e93.7, 74.8e75, 87.3e88.0, and 4.7% in the casesof iron powder, zeolite, slag powder, tire chips, and sand as the

reactive material, respectively (Chung, 2009).

4. Part 3. challenges

4.1. Flow pattern

Efficiency of remediation relies on the sufficient contact andreaction time between contaminants and the remediation agents inmany instances, such as PRB and the injection of MNBs. Sufficientcontact requires uniform flow in a saturated medium, which ismore difficult to achieve as the scale of treatment increases (Fig. 5).Batch test usually achieve thorough mixing and contact, and yiel-ded highest remediation efficiency; meanwhile, the column test issubjected to flow patterns (determining contact) and flow rate(reaction time), and is more specific to the applied conditions (Caoet al., 2019). Efficiency of pilot and full-scale experiments is moreprone to the influence of the flow pattern and flow rate. Flowpattern is influenced by interfacial surface tension, contact angle,flow rate, and soil matrix network properties (Cao et al., 2016Sustainability). Therefore, carefully examining the flow pattern isof crucial importance to engineering applications.

4.2. Multi-layer PRB

Multiple layers of reactive barriers have gained significantattention during the past few years. Challenges are the membraneeffects and inhibition in-between layers. Reactive materials indifferent layers differ in granular size and gradation (size distri-bution). At the interface of two layers, two failure mechanisms canoccur, namely, clogging and loss of materials (similar to pipingfailure in an earthen dam). Both mechanisms were referred to asmembrane effects. Reactive materials or the products of the reac-tion of the preceding layer could be the contaminant loading to thelatter layer, such as ZVI to zeolite. One logical design of amulti-layerPRB is that the first layer is for reducing, the second layer is for

Fig. 5. Flow pattern examples: (a) Fingering patterns observed during water infiltration into the layered sand (Rezanezhad et al., 2006); (b) Distribution of CO2 (yellow) and salinewater (blue) in the microfluidic chip after CO2 replacing saline water (Zheng et al., 2017); (c) Displacement pattern produced by injecting glycerin solution into Ottawa F110 sand(Zhang, 2012); (d) Non-uniform capillary rise in glass beads (Nie et al., 2019). Inset: directions of flow.


oxidizing, and the last layer is for adsorption.

5. Part 4 summary

This paper reviewed the technologies suitable for remediationof soil and groundwater contaminated with municipal solid wasteslandfills. First, an overview of the market around the globe wasgiven, followed by the features of contaminants in an MSW landfillcontaminated site. Then technologies such as permeable reactivebarriers, electrokinetic remediation, microbial remediation, andinjection of either solubilizing agents or micro or nanobubbleswere thoroughly reviewed, with an emphasis on their removalefficiency and on existing projects at lab, pilot or field scale.Furthermore, a design chart was proposed, which is customized forthe remediation of a landfill contaminated site, followed by a fewcase studies implementing the chart. Finally, challenges of existingremediation technologies for MSW landfills were identified, andpotential failure mechanisms were elucidated.

Acknowledgements

This work was sponsored by the National Natural ScienceFoundation of China (Award No.: 51779219). This work was alsosponsored by the Ministry of Science and Technology of China(Award No.: 2018YFC1802300). Financial supports by both the One-Thousand Young Talents Program of the Organization Departmentof the CPC Central Committee and the 100-Talents Program ofZhejiang University to the corresponding author are deeplyappreciated. MOE Key Laboratory of Soft Soils and Geo-environmental Engineering is acknowledged.

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