Accepted Manuscript

Review

Applicability of agricultural waste and byproducts for adsorptive removal of

heavy metals from wastewater

T.A.H. Nguyen, H.H. Ngo, W.S. Guo, J. Zhang, S. Liang, Q.Y. Yue, Q. Li, T.V.

Nguyen

PII: S0960-8524(13)01375-8

DOI: http://dx.doi.org/10.1016/j.biortech.2013.08.124

Reference: BITE 12323

To appear in: Bioresource Technology

Received Date: 14 July 2013

Revised Date: 19 August 2013

Accepted Date: 20 August 2013

Please cite this article as: Nguyen, T.A.H., Ngo, H.H., Guo, W.S., Zhang, J., Liang, S., Yue, Q.Y., Li, Q., Nguyen,

T.V., Applicability of agricultural waste and byproducts for adsorptive removal of heavy metals from wastewater,

Bioresource Technology (2013), doi: http://dx.doi.org/10.1016/j.biortech.2013.08.124

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Applicability of agricultural waste and byproducts for adsorptive 1 removal of heavy metals from wastewater 2

3 T. A. H. Nguyen a, H. H. Ngo a*, W. S. Guo a, J. Zhang b, S. Liang b, Q. Y. Yue b, Q. Lib, T. V. Nguyen a 4

5 a Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, 6 University of Technology, Sydney, Broadway, NSW 2007, Australia 7 b Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental 8 Science & Engineering, Shandong University, Jinan 250100, PR China 9 10 * Corresponding author: School of Civil and Environmental Engineering, University of Technology, 11 Sydney (UTS), P.O. Box 123, Broadway, NSW 2007, Australia. Tel.: +61 2 9514 2745; Fax: +61 2 9514 12 2633. E-mail address: ngohuuhao121@gmail.com 13 14 ABSTRACT 15 16 This critical review discusses the potential use of agricultural waste based biosorbents 17

(AWBs) for sequestering heavy metals in terms of their adsorption capacities, binding 18

mechanisms, operating factors and pretreatment methods. The literature survey 19

indicates that AWBs have shown equal or even greater adsorption capacities compared 20

to conventional adsorbents. Thanks to modern molecular biotechnologies, the roles of 21

functional groups in biosorption process are better understood. Of process factors, pH 22

appears to be the most influential. In most cases, chemical pretreatments bring about an 23

obvious improvement in metal uptake capacity. However, there are still several gaps, 24

which require further investigation, such as (i) searching for novel, multi-function 25

AWBs, (ii) developing cost-effective modification methods and (iii) assessing AWBs 26

under multi-metal and real wastewater systems. Once these challenges are settled, the 27

replacement of traditional adsorbents by AWBs in decontaminating heavy metals from 28

wastewater can be expected in the future. 29

30

Keywords: Agricultural waste based biosorbents; Biosorption capacity; Heavy metals; 31 Removal efficiency 32 33 34 35 36 37 38 39

1. Introduction 1

Heavy metals in the aquatic medium may originate from wastewater of many 2

industries, such as batteries, tanneries, electrical, electroplating, fertilizers, pesticides, 3

mining, refining ores, etc. (Banerjee et al., 2012; Manzoor et al., 2013; ). Due to their 4

hazardous effects, persistency and accumulation tendency, heavy metals can pose a risk 5

to the human and environmental health (Kumar et al., 2012; Marin et al, 2010). The 6

exposure to heavy metals can cause damage to many parts of human bodies, even at 7

very low concentrations. Therefore, the removal of heavy metals from aqueous 8

solutions is of extreme importance. The United States Environmental Protection Agency 9

(USEPA) has set up the maximum contamination levels (MCLs) of heavy metals for 10

surface or groundwater to be used in the drinking supply. Table 1 describes the typical 11

poisoning symptoms and MCLs of the most common heavy metals (Barakat, 2011). 12

Table 1 13

14

Numerous technologies have been developed for heavy metal decontamination. 15

Traditional treatment processes include precipitation, ion exchange, membrane 16

filtration, electroplating, adsorption (Banerjee et al., 2012; Chowdhury and Saha, 2011). 17

These methods represent significant demerits, such as high chemical and energy 18

requirements, hazardous sludge formation, low efficiency when heavy metals 19

concentration below 100 mg/L, high cost at large scale (Marin-Rangel et al., 2012; 20

Mishra et al., 2012). Likewise, high price and limited reusability are key problems, 21

hindering the widespread application of activated carbon, a commonly used adsorbent in 22

heavy metal treatment (Turan and Mesci, 2011). In that context, biosorption has 23

emerged as a promising method, with such advantages as (1) high efficiency even with 24

low metal concentrations, (2) low cost, (3) no additional nutrients requirements, (4) easy 25

operation, (5) potential metal recovery, and (6) without detrimental effects on the 1

environment (Jiménez-Cedillo et al., 2013; Manzoor et al., 2013; Mishra et al., 2010). A 2

wide variety of agricultural waste and byproducts has been explored for the elimination 3

of heavy metals. This article aims at evaluating the feasibility of using AWBs for heavy 4

metal decontamination based on (1) heavy metals adsorption capacities of AWBs; (2) 5

the effects of operating parameters for process optimization; (3) biosorption 6

mechanisms and (4) modification methods for producing better bio-sorbents. We have 7

included mostly recent studies on heavy metal elimination using AWBs in this review. 8

Table 2 9

10

2. The application of AWBs for heavy metal removal 11

2.1. Background 12

AWBs may be different parts of plant, such as bark, stem, leaves, root, flower, fruit 13

biomass, husk, hull, skin, shell, bran and stone. AWBs mainly compose of cellulose, 14

hemi-cellulose and lignin which have a high content of hydroxyl groups. Consequently, 15

they have good abilities to attach heavy metals (Okoro and Okoro, 2011). Besides, 16

AWBs contain a variety of other functional groups, such as acetamido, carboxyl, 17

phenolic, structural polysaccharides, amido, amino, sulphydryl carboxyl groups, 18

alcohols and ester. These functional groups substitute hydrogen ions for metal ions in 19

solution or donation of an electron pair to form complexes with the metal ions in 20

solutions. Due to abundant binding groups, AWBs could be an enormous potential 21

source of adsorbent materials for decontaminating heavy metals from wastewater 22

(Jiménez-Cedillo et al., 2013; Marin-Rangel et al., 2012; Zafar et al., 2007 ). 23

24

2.2. Advantages of using AWBs as biosorbents 25

There are more and more studies using AWBs to replace current conventional 1

systems in decontaminating heavy metals because of their significant merits. The main 2

advantage of AWBs over other conventional adsorbents is their strong affinity and high 3

selectivity toward heavy metals due to the abundant availability of binding groups on 4

the AWBs surface (Banerjee et al., 2012). In addition, AWBs are usually of low cost 5

because of being generated from easy acquiring, abundant, agricultural origin materials 6

(Marin-Rangel et al., 2012). Furthermore, AWBs can be easily processed, applied and 7

recovered without adverse impacts on the environment (Wan Ngah and Hanafiah, 8

2008). These characteristics of AWBs may probably play critical roles in industrial 9

applications, and thus making AWBs become superior to conventional adsorbents. Last 10

but not least, the recycle of agricultural wastes and byproducts for the purpose of heavy 11

metal treatment is believed to reduce wastes in an eco-friendly way. Hence, it agrees 12

well with concepts of effective, innovative and sustainable waste management (Okoro 13

and Okoro, 2011). 14

2.3. Selection of AWBs 15

Searching for good AWBs among various agricultural wastes and by-products is 16

not an easy work for biosorption researchers. In an attempt to make this task easier, 17

researchers have done much work in building selection criteria. According to Park et al. 18

(2010), availability and cheapness are key factors in choosing AWBs for practical 19

application. Based on these criteria, they proposed agro-industrial wastes and 20

biomaterials as the potential sources of AWBs. On the other hand, Chojnacka (2010) 21

argues that adsorption capacity should be considered as the decisive factor in the 22

selection process of AWBs. This result is consistent with that revealed by Wang and 23

Chen (2010). They suggested that only some bio-sorbents with sufficiently high binding 24

capacity and selectivity for heavy metals are suitable for use in a full-scale biosorption 25

process. Most researchers agree that good AWBs need to meet several requirements, 1

including abundant availability, high cost-effectiveness, easy desorption, high 2

regeneration capability, negligible release of unexpected compounds into aqueous 3

solutions. 4

2.4. Comparison of adsorption capacity of AWBs with conventional adsorbents 5

Some studies have made comparisons between biosorbents based on the removal 6

percentages. However, this could lead to misleading conclusions as removal efficiency 7

does not always reflect exactly the adsorption capability of biosorbents. For example, 8

Aman et al. (2008) reported that one gram of potato peel charcoal could remove 99.8% 9

of the copper from the solution of 150 mg/L (100 ml) at pH 6.0 with a shaking time of 10

20 min. However, this biosorbent hold a relatively low maximum adsorption capacity 11

(0.3877 mg/g). Similar observation was reported in a research conducted by Saka et al. 12

(2012). It was found that Palmyra palm fruit seed showed higher Pb(II) removal 13

percentage (100%) than onion skins (93%). Nevertheless, the adsorption capacity for 14

Pb(II) of Palmyra palm fruit seed (24.6 mg/g) was found far lower than that of onion 15

skins (200 mg/g). For that reason, in searching for a ‘good’ biosorbent, comparison 16

should be made based on the maximum adsorption capacity (qmax) rather than on 17

percentage removal (%) of heavy metals. 18

Table 3 19

20

As shown in Table 3, the adsorption capacities of AWBs vary significantly. The 21

influential factors include types of crop residues, elements of heavy metals, 22

pretreatments of AWBs and especially operating conditions. AWBs tended to prefer 23

some heavy metals to others. Kelly-Vargas et al. (2012) reported that lemon peel and 24

orange peel demonstrated the adsorption capacities for Cu and Pb 48% and 15% higher 25

than banana peel, respectively. In contrast, the Cd uptake by banana was higher than 1

that of lemon peel and orange peel 82% and 57%, respectively. A study, conducted by 2

Mosa et al. (2011) released that the removal efficiency of heavy metals decreased in the 3

order cotton stalks > maize stalks> rice straw. They attributed the highest removal 4

percentage by cotton stalks to its highest concentration of cellulose, hemicellulose, and 5

lignin as compared to other crop-residues. Osman et al. (2010) reported that rice hull 6

showed the highest efficiency in sequestering zinc, cadmium and iron among 7

biosorbents investigated. For example, the removal efficiencies by rice hull, sawdust, 8

sugarcane bagasse and wheat straw were 98.15, 96.90, 93.00 and 91.19%, respectively. 9

They suggested that the higher adsorption capacity of rice hull than other sorbents for 10

removal of heavy metals was probably due to the presence of silanol (SiOH) groups in 11

structure of rice hull and higher surface area of rice hull. 12

As can be seen in Table 3, most AWBs even without any chemical pretreatments 13

showed better adsorption capacities for heavy metals as compared with conventional 14

adsorbents. For instance, the Cu uptake capabilities of cortex banana, orange, lemon 15

waste, garden grass, palm oil fruit shell, etc. in the raw form (ranging from 27.68 to 16

70.40 mg/g) were similar to or even higher than that of ion exchange resins (26.73 17

mg/g). Some other kinds of original AWBs, such as tamarind seed, watermelon shell or 18

rose petal waste exhibited the adsorption capacity for Cu(II) ions 210%, 315% and 19

364% higher than that of above ion exchange resins. This can be attributed to the 20

abundant availability of binding sites on the AWBs, which enhances the retention of 21

heavy metals onto AWBs surface (Jiménez-Cedillo et al., 2013; Marin-Rangel et al., 22

2012). Particularly, cashew nut shell modified with H2SO4 exhibited an extremely high 23

adsorption capacity (406.6 mg/g) for Cu(II) ions. The remarkable enhancement in 24

adsorption capacity for Cu(II) using H2SO4 is probably due to elimination of competing 25

cations and the increase in surface area as well as the porosity on the biosorbent surface 1

(Boota et al., 2009; Lasheen et al., 2012; Osman et al., 2010). Some AWBs showed 2

very high affinity toward heavy metals in the natural form. For example, the adsorption 3

capacity for Cr(VI) ions of the raw coir pith was relatively high (165 mg/g). However, 4

coir pith grafted with acrylic acid even exhibited much better Cr(VI) uptake capability 5

(196 mg/g) (Suksabye and Thiravetyan, 2012). Obviously, chemical pretreatments can 6

vastly enhance adsorption capacities of ABWs. These effects will be further discussed 7

in Section 5 later. Similar observations can be reported for the remaining heavy metals 8

in Table 3, such as As, Cd, Cr, Pb, Zn. 9

The experimental conditions in the literature were very diverse. Therefore, it would 10

be challenging to identify the best AWBs for this purpose (Sahmoune et al., 2011). Saka 11

et al. (2012) claimed that AWBs with loading capacities ≥90 mg/g should be considered 12

as good biosorbents. Based on this criteria, this present review introduces the promising 13

AWBs including prawn shell activated carbon (Arulkumar et al., 2012); watermelon 14

shell/ rind (Banerjee et al., 2012; Liu et al., 2012); rose petals waste (Bhatti et al., 2011; 15

Manzoor et al., 2013); orange peel (Feng et al., 2011; Liang et al., 2009); durian shell 16

waste (Kurniawan et al., 2011); Cedrus deodara sawdust (Mishra et al., 2012); onion 17

skins (Saka et al., 2011); cashew nut shell (Senthil Kumar et al., 2012); coir pith 18

(Suksabye and Thiravetyan, 2012). 19

20

3. Mechanisms of metal uptake on AWBs 21

It is essential to investigate the adsorption mechanisms to obtain understandings 22

about the process, which are useful for improvement of the system in the future (Park et 23

al., 2010). Several factors are found to affect the heavy metal adsorption by AWBs, 24

including types of AWBs, properties of metal solution chemistry and environmental 25

conditions. As a result, the actual mechanism of the metal biosorption is not fully 1

understood, though many mechanisms have been proposed for the retention of heavy 2

metals onto AWBs (Figure 1). 3

Figure 1 4

5

It is likely that various mechanisms can operate simultaneously to varying degrees. 6

Netzahuatl-Muñoz et al. (2012) examined mechanisms involved the removal of Cr(III) 7

and Cr(VI) by Cupressus lusitanica bark. Ion exchange and electrostatic interaction 8

were considered as the principal mechanisms for Cr(III) biosorption while Cr(VI) 9

biosorption was supposed to occur in four reaction steps: (1) formation of Cr(VI) 10

complexes, (2) reduction of Cr(VI) to Cr(III), (3) formation of carboxyl groups and (4) 11

interaction of Cr(III) with carboxyl groups to form Cr(III) - carboxylate complexes. 12

Feng and Guo (2012) reported that ion exchange might be the dominant mechanism in 13

the removal of Cu(II), Zn(II) and Pb(II) using orange peel. The results of an X -ray 14

fluorescence for modified orange peel (SCOP) showed that the adsorption process 15

followed an ion exchange mechanism between Ca(II) from SCOP and metals in 16

solution. Njoku et al. (2011) also claimed that Pb(II) and Cu(II) biosorption on Cocoa 17

pod husk followed an ion exchange mechanism. Similar results were obtained by Taha 18

et al. (2011) in case of removal of Pb(II) Cd(II) and Zn(II) using potato peel. A decrease 19

in the solution pH values, after shaking heavy metal solution with potato peel, suggested 20

that ion exchange might be one of the mechanisms. They attributed the high adsorption 21

capacity of potato peels to active functional groups such as carboxylic, phenolic and 22

hydroxyl groups. These findings agree with previous studies performed by Panda et al. 23

(2008). In contrast, Sen et al. (2011) reported that Hg(II) adsorption on Rambai leaves 24

was a two step process, a rapid adsorption of metal ion to the external surface followed 1

by intra-particle diffusion into the interior of adsorbent. 2

Due to the complicated nature of biosorption mechanisms, the combination of 3

modern methods e.g. FTIR (Fourier Transform Infra Red), SEM (Scanning Electron 4

Microscopy), EDX (Energy Dispersive X-Ray), TEM (Transmission Electron 5

Microscopy), etc. and traditional methods e.g. titration are essential. By employing 6

FTIR, SEM, EDX methods, Witek-Krowiak et al. (2013) discovered that the dominant 7

mechanisms for the biosorption of Cr(III) and Cu(II) onto soybean meal waste included 8

(1) ion exchange, (2) chelation by carboxyl and hydroxyl groups, and (3) precipitation. 9

Equally, via the desorption results, the Dubinin - Radushkevich isotherm, and energy 10

parameter (E), Ofomaja et al. (2010) proposed that ion - exchange was dominant 11

mechanism in the removal of Cu(II) and Pb(II) using KOH treated pine cone powder. 12

Vázquez et al. (2012) highlighted the role of functional groups in the biosorption of 13

Cu(II), Zn(II), Pb(II) and Cd(II) into Chestnut (Castanea sativa) shell. They reported 14

that there was a decline in adsorption capacity of Cd(II), Cu(II), and Zn(II) up to 32.8, 15

58.5 and 65.3% respectively due to the chemical blocking of carboxyl groups of 16

pre-treated Chestnut (Castanea sativa) shell. This results show the important role of 17

carboxyl groups in the biosorption process. Similarly, the blockage of hydroxyl groups 18

reduced the adsorption percentages for Cd(II), Cu(II), and Zn(II) by 30.9, 27.5 and 19

46.1%, respectively. These decreases confirm the role of OH groups in Cd(II), Cu(II), 20

and Zn(II) adsorption by the alkali pre-treated chestnut shell. Conversely, carboxyl and 21

hydroxyl groups blocking of chestnut shell had no significant effect on Pb(II) 22

adsorption. Kumar et al. (2012) also found that a variety of functional groups, such as 23

carboxyl and hydroxyl groups, involved in the binding mechanisms of Cd(II) by cashew 24

nut shell. Based on FTIR spectra, Lasheen et al. (2012) indicated the involvement of 25

carboxylic groups in Pb(II) ions adsorption process using chemically modified orange 1

peel. With the help of FTIR, Gutha et al. (2011) reported that hydroxyl, amine, 2

carboxyl, and carbonyl functional groups were responsible for Ni(II) biosorption onto 3

Caesalpinia bonducella seed powder, while Feng et al. (2011) revealed that carboxyl 4

and hydroxyl groups involved in the biosorption of the metal ions by grafted 5

polymerization-modified orange peel. 6

It is obvious that hydroxyl, carboxyl, phosphate, hydroxyl, amino and thio groups 7

on AWBs cell walls play major roles in binding heavy metals. These functional groups 8

make AWBs be capable of binding heavy metals by changing their hydrogen ions for 9

metal ions or giving an electron pair to form complexes with the metal ions (Kumar et 10

al., 2011; Wang and Chen, 2010). However, the existence of these functional groups on 11

the surface of AWBs does not guarantee an effective removal of heavy metals as the 12

biosorption process may be influenced by several other factors, such as (1) the number 13

of active sites, (2) accessibility of the sites, (3) chemical state of the sites and (4) affinity 14

between the sites and the target metal ions (Park et al., 2010). 15

16

4. Parameters affecting heavy metal biosorption 17

The biosorption process of heavy metals from wastewater might be influenced by 18

several physical and chemical factors, such as pH, temperature, initial heavy metal 19

concentration, biosorbent dose, biosorbent size, ionic strength, co-ions, etc. These 20

factors determine the overall biosorption through affecting the uptake rate, selectivity 21

and amount of heavy metals removed. Extensive research has been undertaken to 22

investigate effects of these operating parameters. This section is dedicated to briefly 23

discuss the effects of these process factors. 24

4.1. Influence of pH 25

Among process factors, pH seems to play a significant role in controlling the 1

biosorption of heavy metals. pH values can affect the surface charge of AWBs, the 2

degree of ionization and speciation of heavy metals, the competition of the metal ions 3

with coexisting ions in solution (Park et al., 2010). This pH dependency could be 4

explained by the involvement of functional groups in metal uptake and metal chemistry 5

(Kumar et al., 2012). As a rule, as solution pH increases, the biosorptive removal of 6

cationic metals increases, whereas that of anionic metals decreases. At lower pH, the 7

overall surface charge of AWBs will be positive. The H+ ions compete effectively with 8

the metal cations causing a decrease in biosorption capacity. When pH values increase, 9

the AWBs surface becomes increasingly negatively charged which favors the metal ions 10

uptake due to electrostatic interaction. At very high pH, the biosorption stops and the 11

hydroxide precipitation starts (El-Sayed et al., 2011; Njoku et al., 2011; Taha et al., 12

2011). 13

Similar tendencies were found in biosorption processes using diverse AWBs. Giri 14

et al. (2012) studied the effect of pH on the removal of Cr (VI) using Eichhornia 15

crassipes root activated carbon. They reported that the Cr(VI) adsorption efficiency 16

increased from 41.22% to 85.52% for 10 mg/L, 45.34% to 89.23% for 50 mg/L and 17

50.23% to 92.24% for 100 mg/L with the increase of pH from 1.5 to 4.5. Taha et al. 18

(2011) investigated the role of pH in the adsorption Pb(II), Cd(II) and Zn(II) ions using 19

potato peels. They reported that the removal efficiency increased with increasing pH 20

values from 2 to 6; after pH 6, there was a decrease in metal ion removal. Similar trend 21

was reported by Reddy et al. (2011) in case of removal Ni(II) with Moringa oleifera 22

bark. They found that, under highly acidic conditions, the removal percentage of Ni(II) 23

was very small. The adsorption increased with the increase in pH from 3.0 to 6.0 and 24

then decreased in the range of pH between 7.0 and 8.0. These results agree with those 25

reported by Feng et al. (2011). They revealed that the percent biosorption for Pb(II), 1

Cd(II) and Ni(II) by grafted copolymerization-modified orange peel was minimal at pH 2

2.0 and increased with increasing pH from 2.0 to 5.5. 3

For optimization of biosorption processes, a large part of biosorption studies are 4

devoted to investigating the optimum pH values. Table 4 summarizes the findings on 5

optimum pH values in various heavy metals - AWBs systems. It can be seen that the 6

optimum pH values are dependent on types of ABWs. Aman et al. (2008) reported that 7

the optimum pH for Cu(II) - potato peel adsorption system was 6.0. The pH dependence 8

of metal uptake might be associated with the surface functional groups on AWBs cell 9

walls. Different types of ABWs contain various functional groups on their cell walls 10

thereby reaching the maximum adsorption capacity at different pH values. In order to 11

avoid the precipitation of metal hydroxides, several studies have been conducted to 12

identify pH values, at which metal hydroxide precipitation starts. Aman et al. (2008) 13

reported that precipitation of Cu(OH)2 took place at pH greater than 6.0 in case of 14

removal of Cu(II) by potato peels. 15

Table 4 16

17

4.2. Influence of temperature 18

Biosorption researchers have performed a large number of studies on the effects of 19

temperature on metal uptake. The change in solution temperature affects not only 20

diffusion rate of metal ions but also the solubility of metal ions (Park et al., 2010). 21

Depending on surface functional groups of a given AWB, temperature has a given 22

impact on the adsorption capacity. However, it is a common conclusion of many studies 23

that the influence of temperature is to a limited extent and only in a certain temperature 24

range (Sahmoune et al., 2011). The biosorption process can be affected by temperature 1

in different ways depending on the exothermic or endothermic nature of the process. 2

Many researchers reported that biosorption processes are exothermic which means 3

that adsorption capacity is inversely proportional to the temperature (Sahmoune et al., 4

2011). Kumar et al. (2012) found that the biosorption of Cd(II) by cashew nut shell 5

decreased from 80.13% to 74.32% with the rise in temperature from 30 to 60oC. They 6

attributed this to the decrease in surface activity of AWBs. Similar trend was noticed by 7

El-Sayed et al. (2011) in case of Zn(II), Cd(II) and Mn(II) biosorption onto maize 8

stalks. The biosorption percentage decreased from 52% to 28% for Zn(II) ions, from 9

34% to 16% for Cd(II) ions and from 39% to 13% for Mn(II) ions as the temperature 10

increased from 25 to 550C. The authors explained this trend by the damage of active 11

adsorption sites of AWBs or increasing number of metal ions escaped from the AWBs 12

surface to the solution. The same behavior can be detected in a research performed by 13

Boota et al. (2009) sequestering Cu(II) and Zn(II) ions by Citrus reticulate. 14

In contrast, in their 2010 review paper, Park et al. (2010) argued that biosorption of 15

heavy metals was endothermic in nature. They suggested that higher temperature 16

improved the elimination of heavy metals owing to an increase in its surface activity 17

and kinetic energy. Giri et al. (2012) reported the similar trend in case of adsorption 18

Cr(VI) by the activated carbon originated from Eichhornia crassipes roots. They 19

explored that the percentage removal of Cr(VI) increased from 79.24% to 92.24% as the 20

temperature increased from 25 to 550C. In the same way, García-Rosales and 21

Colín-Cruz (2010) observed that the adsorption of Pb(II) and Cd(II) by the stalk sponge 22

of Zea mays increased 1.1- 1.8 times with increasing temperature from 200C to 400C. 23

Banerjee et al. (2012) reported similar observation in case of adsorption of Cu(II) by 24

watermelon shell. They attributed this trend to either increase in number of available 25

active sites or decrease in the boundary layer thickness surrounding the AWBs. 1

Nevertheless, Park et al. (2010) claimed that high temperature might result in the 2

physical damage of AWBs. For this reason, the room temperature is commonly used in 3

most biosorption experiments. 4

4.3. Effect of initial concentration 5

Heavy metal ions can transport from the solution to the surface of AWBs owing to 6

a driving force made by the initial metal concentration (Sahmoune et al., 2011; Taha et 7

al., 2011). Researchers have carried out several studies to clarify the effect of initial 8

concentration on metal uptake by AWBs. 9

It is a common finding that the maximum adsorption capacity of a specific AWB 10

increases as the initial metal concentration increases. Kumar et al. (2012) reported that 11

there was a rise in Cd(II) adsorption capacity, from 2.671 to 11.095 mg/g, as a result of 12

increasing Cd(II) concentration from 10 to 50 mg/L. Also, Reddy et al. (2010) revealed 13

that the Pb(II) uptake by Moringa oleifera leaves increased from 12 to 23mg/g as the 14

initial concentration of Pb(II) increased from 10 to 40mg/L. Wang and Chen (2010) 15

explained this by a higher collision between metal ions and AWBs. 16

Conversely, the increase of initial metal concentration usually leads to a decrease 17

of the removal efficiency. Kannan and Veemaraj (2010); Kumar et al. (2012) attributed 18

this behaviour to the saturation of adsorption sites on AWBs surface, whilst Kumar et 19

al. (2011) explained this by the lower rate of transporting metal ions from solution to 20

AWBs surface. Kumar et al. (2012) reported that an increase in Cd(II) concentration, 21

from 10 to 50mg/L, resulted in a decrease in Cd(II) removal percentage from 80.13% to 22

66.57%. Ashraf et al. (2011) reported a similar trend for the case of removing Pb(II), 23

Cu(II), Zn(II) and Ni(II) by the banana peel (Musa sapientum). At the highest metal 24

concentration (150 mg/L), the removal efficiencies of Cu(II), Pb(II), Zn(II) and 25

Ni(II) were 92.52%, 79.55%, 63.23% and 68.10% while at the lowest metal 1

concentration (25 mg/L), the removal percentages of these metals were 94.80%, 2

86.81%, 84.63% and 82.36%, respectively. Kannan and Veemaraj (2010) reported that 3

the percentage removal of Cd(II) by jack fruit seed (JFS) decreased from 66.28% to 4

22.43% with an increase in initial Cd(II) concentration from 10 to 100 ppm. 5

Conversely, Giri et al. (2012) reported that as the initial Cr(VI) concentration increased 6

from 10 mg/L to 100 mg/L, the removal of Cr(VI) by Eichhornia crassipes root 7

activated carbon increased from 77.22% to 92.24%. They suggested that the initial 8

metals concentration provided a driving force to overcome mass transfer resistance of 9

Cr(VI) ions between the aqueous and solid phase. Hence, further work is required to 10

elucidate this disagreement in the literature. 11

4.4. Effect of biosorbent dosage 12

Several researchers report that the removal percentages of metal ions increase with 13

increasing AWBs dosages. Kumar et al. (2012) found that the Cd(II) removal increased 14

rapidly as cashew nut shell dosage increased. The maximum percentage removal of 15

Cd(II) ion was 75.35% at the cashew nut shell concentration of 3 g/L. They attributed 16

this behaviour to the higher number of available adsorption sites. Similar observations 17

were obtained by Gala and Sanak-Rydlewska (2011). On the other hand, maximum 18

adsorption capacities of AWBs are found to decrease with increasing biosorbent doses. 19

Boota et al. (2009) revealed that there was a remarkable decrease in the adsorption 20

capacities of Cu(II) and Zn(II) as Citrus reticulata dose increased. They ascribed this 21

effect to overlapping of adsorption sites leading to a decrease in the total surface area. 22

4.5. Effect of ionic strength 23

Real industrial wastewater contains not only heavy metals but also other metal 24

ions, such as Na+, K+, Ca2+, Mg2+. Therefore, NaCl, NaNO3, KCl, MgCl2, CaCl2 are 25

usually added into heavy metal solutions to investigate effects of ionic strength on the 1

biosorption of heavy metal ions. El-Sayed et al. (2011) reported that a rise in ionic 2

strength led to a decrease in the metal uptake capacity. They ascribed this to the 3

decrease in the activity of metal ions and the increase in concentration of competing 4

cations. Njoku et al. (2011) observed that the effect of Ca2+ on the biosorption of the 5

metal ions was more significant than that of Na+. They explained this by the fact that the 6

divalent Ca2+ ion had a greater affinity to the active sites and thereby might compete 7

more strongly for binding sites than the monovalent Na+ ions. Conversely, Reddy et al. 8

(2010) explored that the presence of common metal ions like Na+, K+, Ca2+ and Mg2+ 9

had no significant effects on the biosorption of Pb(II) by Moringa oleifera leaves. 10

4.6. Effect of co-ions 11

The real wastewaters generally contain multi heavy metals rather than single metal. 12

The presence of one heavy metal may hinder the adsorption of other heavy metals. Thus, 13

the investigation of effects of co-ions is useful for enhancing the application of 14

biosorbents at industrial scales. Diverse effects of co-exiting ions on the biosorption of 15

heavy metals are reported by different researchers. Goyal and Srivastava (2009) found 16

that the removal percentages of heavy metals by Zea mays in single metal solutions (Pb 17

87.34%, Cd 79.36%, Ni 71.98% and Cr 76.43%) were higher than those in a 18

multi-metal solution (Pb 81.21%, Cd 73.72%, Ni 64.03% and Cr 68.91%). They 19

attributed this effect to the competition between cations. In the contrary, 20

García-Mendieta et al. (2012) reported that the removal percentages of Mn and Fe from 21

a binary system were similar to the values found in single systems. This behavior 22

indicated that Fe and Mn did not compete for the adsorption sites on the green tomato 23

husk. Chiban et al. (2012) studied the removal of arsenate using W.frtescens and 24

C.rhizome plants. They found that the presence of Mg2+, Cd2+, Cu2+ and Zn2+ ions in the 25

metal solution had no significant effects, whereas HPO42- interfered strongly. They 1

found that the effect of competing anions on arsenate adsorption reduced in the order 2

HPO42-> SO4

2-> Cl-> NO3-. The authors concluded that the interfering effects on the 3

arsenate biosorption became stronger with the increasing valence of competing anions. 4

Ofomaja et al. (2010) revealed that there was mutual interference effect in the 5

adsorptive Cu-Pb system, using pine corn powder treated with KOH. However, the 6

competing effect was stronger for the Cu-Pb system compared to the Pb-Cu system. The 7

biosorption studies in binary and ternary systems are essential to promote the real 8

application of biosorbents at large scales. However, little information on this content is 9

published. 10

4.7. Selectivity 11

A given AWB tends to prefer some heavy metals to the others. Vimala and Das 12

(2009) found that oyster mushroom (Pleurotus platypus) showed the highest adsorption 13

capacity for Cd, whereas button mushroom (Agaricus bisporus) exhibited maximum 14

metal uptake for Pb(II). Milky mushroom (Calocybe indica) showed the lowest metal 15

uptake for both metals. Mosa et al. (2011) reported that cotton stalks showed high 16

affinity to Pb(II), but extremely low affinity to Mn(II). Taha et al. (2011) found that the 17

adsorption capacity by potato peels reduced in the order Pb(II) > Cd(II) > Zn(II), whilst 18

the hydrated ionic radii increased in the order Pb(II) < Cd(II) < Zn(II). They concluded 19

that the removal percentages of metal ions increased with their decreasing hydrated 20

ionic radii. Jiménez-Cedillo et al. (2013) explored that iron-modified non-pyrolyzed 21

parsley biomass (PCFe) eliminated 16 times more As(V) than As(III), whereas 22

iron-modified pyrolyzed parsley biomass (PCTTFe) removed 2.5 times more As(III) 23

than As(V). Based on these results, they concluded that PCFe was selective for As(V), 24

while the removal of As(V) was favored by PCTTFe. 25

4.8. Effect of particle size 1

The particle size of AWBs can influence their adsorption capacities due to the 2

change in total surface area which is necessary for metal adsorption. Banerjee et al. 3

(2012) reported that the smaller particles of watermelon shell showed higher removal 4

efficiency of Cu(II). These results are consistent with findings reported by other 5

authors. Kelly-Vargas et al. (2012) explored that removal percentage of metal ions by 6

banana cortex (Musa paradisiaca) with 1 mm of particle size was higher than that of 2 7

mm of particle size up to 12%. In the same way, Kannan and Veemaraj (2010) observed 8

that the removal of Cd(II) increased from 10.07% to 53.16% with decreasing particle 9

size of Jackfruit seed carbon, from 250 µm to 90 µm. They attributed this to the increase 10

in the available surface area. Boota et al. (2009) reported similar findings for 11

sequestering Cu(II) and Zn(II) using Citrus reticulata. In contrast, Taha et al. (2011) 12

explored that a decrease in particle size of potato peels did not profoundly change the 13

adsorption capacities of Pb(II) Cd(II) and Zn(II). 14

15

5. Effects of pretreatment on heavy metal biosorption 16

Untreated AWBs have been used for eliminating heavy metals from wastewater in 17

many studies conducted by Ashraf et al. (2011), Banerjee et al. (2012), El-Sayed et al. 18

(2011), Gala and Sanak-Rydlewska (2011), Gutha et al. (2011), Kelly-Vargas et al. 19

(2012), Kumar et al. (2012), Liu et al. (2012), Netzahuatl-Muñoz et al. (2012), Njoku et 20

al. (2011), Reddy et al. (2011), Sen et al. (2011), etc. However, the application of 21

untreated AWBs has significant drawbacks such as low adsorption capacity, high 22

release of soluble organic compounds into the solution. The increase in chemical 23

oxygen demand (COD), biological oxygen demand (BOD) and total organic carbon 24

(TOC) may lead to the depletion of dissolved oxygen (DO) concentration in aquatic 25

solutions and consequently this affects aquatic lives. For these reasons, it is highly 1

recommended that AWBs need to be pretreated before being used in biosorption 2

processes (Feng et al., 2011). There is an increasing trend of modifying AWBs to 3

improve the removal efficiency of heavy metals, remove soluble organic compounds 4

and eliminate coloration of the solutions (Wan Ngah and Hanafiah, 2008). 5

Figure 2 6

7

The modification methods include physical modifications, chemical modifications 8

and other methods (Figure 2). Physical modifications are usually considered being 9

simple and inexpensive. However, they are not widely used because of their low 10

effectiveness. Conversely, chemical modifications are preferred, due to their simplicity 11

and efficiency (Park et al., 2010). Modifying agents can be classified as bases, mineral 12

and organic acids, organic compounds, oxidizing agents, etc. However, the most 13

commonly used chemicals are acids and bases. Other modification methods include 14

enhancement of binding groups, elimination of inhibiting groups or graft 15

polymerization. 16

Regarding the effects of chemical pretreatments, the literature reveals that, 17

chemically modified AWBs showed better adsorption capacities than unmodified forms 18

(Wang and Chen, 2010). It can be attributed to the higher number of binding sites, 19

better ion-exchange ability, and formation of new functional groups that favor metal 20

uptake. Chemical pretreatments can improve the adsorption capacity of AWBs to 21

various extents. Table 5 summarizes the findings on the effects of AWBs modification 22

methods on metal removal efficiencies. 23

Table 5 24

Bases are found to be effective modifying agents. Feng and Guo (2012) reported 1

that the adsorption capacity of Cu(II), Pb(II) and Zn(II) increased 59.73%, 84.84% and 2

164.38%, respectively as a result of modifying orange peel with sodium hydroxide 3

(NaOH) and calcium chloride (CaCl2). Mosa et al. (2011) revealed that, among 4

different modifying chemicals, NaOH resulted in the highest percentage removal. They 5

explained that the pretreatment with NaOH led to the transformation of methyl esters as 6

inhibiting groups to carboxylate ligands. Consequently, the metal binding capability 7

was substantially improved. 8

The efficacy of different inorganic acids in modification of AWBs has been 9

confirmed by many researchers. Lasheen et al. (2012) explored that Cd(II) uptake 10

capacity increased 61.38% as a result of pretreating orange peel with 0.1M HNO3. They 11

suggested that HNO3 helped to remove excess cations such as K(I), and Ca(II) on the 12

surface of orange peel and thereby reducing the competition between these ions with 13

Cd(II) ions. Similar observation was noticed by Osman et al. (2010) in case of 14

modifying AWBs with 0.1M HNO3. They found that the removal efficiencies of metal 15

ions by modified AWBs (Zn 88.41% - Cd 84.30% - Fe 94.81%) were substantially 16

higher than those of raw forms (Zn 77.34% - Cd 69.78 - Fe 86.36%). They attributed 17

this enhancement to the increasing surface area. These results are in good agreement 18

with those reported by Boota et al. (2009). They found that, among different modifying 19

chemicals, H2SO4 resulted in the highest metal uptake capacities. They suggested that 20

pretreatment with H2SO4 led to a negative charge surface of biomass, increased surface 21

area and porosity of biomass. Similarly, Elangovan et al. (2008) reported that acid 22

pretreatment substantially improved Cr(VI) uptake capacity of mangrove leaves and 23

water lily. In contrary, Manzoor et al. (2013) revealed that, the pretreatment of Rosa 24

bourbonia phyto-biomass with organic acids (i.g. acetic acid, benzoic acid, citric acid) 25

led to a decrease in its sequestering ability of Pb(II) and Cu(II). They attributed this to 1

the damage or occupation of active binding sites by these organic acids. 2

In some instances, organic compounds as modifying agents are found to increase in 3

the metal uptake capacity of AWBs. García-Mendieta et al. (2012) found that the 4

adsorption capacities of Fe(II) and Mn(II) were slightly improved as the result of 5

pretreating green tomato husk with 0.2% formaldehyde. Bhatti et al. (2011) reported 6

that pretreatments of red rose biomass with methanol and polyethyleneimine + 7

glutraldehyde brought about highest adsorption capacities for Pb(II) and Co(II), 8

respectively. 9

Vast improvements in the adsorption capacity of AWBs can be obtained by 10

modifying their functional groups. Suksabye and Thiravetyan (2012) reported that the 11

Cr(VI) adsorption capacity of acrylic acid grafted coir pith (196 mg/g) was markedly 12

higher than that of the natural coir pith (165 mg/g). Based on the FTIR results, they 13

ascribed this to the increase in the carbonyl groups (C=O) on the surface of coir pith, 14

resulted from grafting with acrylic acid. Feng et al. (2011) found that grafted 15

copolymerization of orange peel increased the metal uptake of Pb(II), Cd(II) and Ni(II) 16

up to 4.2 - 4.6 - 16.5 times, respectively. They suggested that modified orange peel had 17

higher ion exchange and chelating capacity than raw orange peel. Goyal and Srivastava 18

(2009) reported that the metal uptake capacity was proportional to the number of 19

functional groups on the AWBs. They revealed that adding carboxyl groups into 20

nitrogen ligand of Zea mays cob powder led to a considerable enhancement (5-15%) in 21

adsorption efficiency of Pb(II), Cd(II), Ni(II) and Cr(III). These results agree with a 22

previous study by Panda et al. (2008). They explored that the adsorption capacity of 23

Cd(II) and Ni(II) by functionalized husk of Lathyrus sativus was 50% higher than that 24

of the pristine one. 25

Cationization of biosorbents by impregnation of biomaterials with metal solutions 1

was supposed to improve clearly their adsorption abilities towards heavy metals in the 2

form of anions, e.g. As(V), Cr(VI). Majumdar et al. (2013) reported that the maximum 3

removal percentage of Cr(V) using untreated rice husk silica-carbon (RHSC-U) was 4

74.7%, whereas for the iron impregrated rice husk silica-carbon, it reached up to 85.9%. 5

Similarly, the impregnation of silica-carbon with 1% Fe solution resulted in an increase 6

by 19% in the As(V) pick up percentage. Among different metals (i.e. Zn, Fe, and Ag) 7

which were used for this purpose, Ag led to the highest enhancement (20%), followed 8

by Fe (19%) and Zn (14%). This demonstrates the need of further exploration of 9

specific metals for impregnation. 10

As a rule, the adsorption capacity of metal ions by AWBs can be remarkably 11

enhanced with proper chemical pretreatments. Nevertheless, this may contain some 12

drawbacks. The chemical pretreatments may increase the cost of treatment, thereby 13

reducing advantages of AWBs over conventional adsorbents (Park et al., 2010). 14

Furthermore, chemical pretreatments may result in biomass loss, thus hindering their 15

use for long term. In this regards, Lasheen et al. (2012) claimed that there was no 16

significant weight loss during protonation of orange peel with 0.1M HNO3. In contrast, 17

García-Mendieta et al. (2012) revealed that the weight loss, which was caused by 18

modifying green tomato husk with 0.2% formaldehyde, was 26.7%. Finally, the use of 19

diverse chemicals for the purpose of AWBs pretreatments may lead to discharging of 20

unexpected compounds into water bodies, e.g. colored organic compounds (Kumar et 21

al., 2011). In contrast, Jiménez-Cedillo et al. (2013) reported that only marginal 22

amounts of iron were released into aqueous solution during the adsorption process of 23

As(III) and As(V) using non-pyrolyzed and pyrolyzed Petroselinum crispum (parsley) 24

biomass modified with FeCl3. However, for similar studies, when biosorbents are 25

impregnated with metals to enhance their removal efficiencies towards anions, the 1

detachment of loading metal should be examined thoroughly. This is because it can not 2

only deteriorate the quality of aqueous solutions but also affect reusability of 3

biosorbents. For that reason, it is very necessary to find out efficient methods to 4

mitigate the adverse effects of pretreatments while enhancing the adsorption capacity of 5

AWBs. 6

7

6. Conclusions and future research directions 8

AWBs can successfully eliminate heavy metals from wastewater. Therefore, 9

AWBs should be further explored for novel, cost-effective and more selective 10

biosorbents. 11

Despite existing limitations, chemical pretreatments of AWBs may result in 12

remarkable enhancement in their metal uptake capacities. Thus, more attention should 13

be paid to improving the pretreatment methods. 14

A majority of biosorption studies have employed AWBs in single metal systems. 15

Hence, further work is required in multi metal systems and real wastewater to make 16

industrial usage of AWBs more feasible. Particularly, there is a need to develop 17

versatile biosorbents for the simultaneous decontamination of several pollutants. 18

19

Acknowledgements 20

This research was supported by Research Theme of Sustainable Water - Wastewater 21

Treatment and Reuse technologies, Centre for Technology in Water and Wastewater 22

(CTWW), School of Civil and Environmental Engineering, University of Technology, 23

Sydney (UTS) and Australian Scholarships for Development in Vietnam (ASDiV). The 24

research collaboration between UTS and Shandong University is also grateful. 25

REFERENCES

1. Aman, T., Kazi, A.A., Sabri, M.U., Bano, Q., 2008. Potato peels as solid waste for the removal of heavy metal copper(II) from waste water/industrial effluent. Colloids and Surfaces B: Biointerfaces 63, 116-121.

2. Anirudhan, T.S., Unnithan, M.R., 2007. Arsenic (V) removal from aqueous solutions using an anion exchanger derived from coconut coir pith and its recovery. Chemosphere 66, 60-66.

3. Araujo, A.L.P., Bertagnolli, C., Silva, M.G.C., Gmenes, M.L., Barros, M.A.S.D., 2013. Zinc adsorption in bentonite clay: influence of pH and initial concentration. Acta Scientiarum Technol. 35, 325-332.

4. Arulkumar, M., Thirumalai, K., Sathishkumar, P., Palvannan, T., 2012. Rapid removal of chromium from aqueous solution using novel prawn shell activated carbon. Chem. Eng. J. 185-186, 178-186.

5. Ashraf, M.A., Wajid, A., Mahmood, K., Maah, M.J., Yusoff, I., 2011. Low cost biosorbent banana peel (Musa sapientum) for the removal of heavy metals. Sci. Res. Essays 6, 4055-4064.

6. Azouaou, N., Sadaoui, Z., Djaafri, A., Mokaddem, H., 2010. Adsorption of cadmium from aqueous solution onto untreated coffee grounds: Equilibrium, kinetics and thermodynamics. J. Hazard. Mater. 184, 126-134.

7. Banerjee, K., Ramesh, S.T., Nidheesh, P.V., Bharathi, K.S., 2012. A novel agricultural waste adsorbent, watermelon shell for the removal of copper from aqueous solutions. Iranica J. Energy Environ. 3, 143-156.

8. Barakat, M.A., 2011. New trends in removing heavy metals from industrial wastewater. Arabian J. Chem. 4, 361-377.

9. Bhatti, H.N., Khadim, R., Hanif, M.A., 2011. Biosorption of Pb(II) and Co(II) on red rose waste biomass. Iran. J. Chem. Eng. 30, 80-88.

10. Boota, R., Bhatti, H.N., Hanif, M.A., 2009. Removal of Cu(II) and Zn(II) using lignocellulosic fiber derived from citrus reticulata (Kinnow) waste biomass. Sep. Purif. Technol. 44, 4000-4022.

11. Chiban, M., Soudani, A., Sinan, F., Persin, M., 2012. Wastewater treatment by batch adsorption method onto micro-particles of dried Withania frutescens plant as a new adsorbent. J. Environ. Manage. 95, 61-65. Chojnacka, K., 2010. Biosorption and bioaccumulation - the prospects for practical applications. Environ. Int. 36, 299-307.

12. Dubey, A., Shiwani, S., 2012. Adsorption of lead using a new green material obtained from Portulaca plant. Int. J. Environ. Sci. Technol. 9, 15-20.

13. Elangovan, R., Philip, L., Chandraraj, K., 2008. Biosorption of chromium species by aquatic weeds: Kinetics and mechanism studies. J. Hazard. Mater. 152, 100-112.

14. El-Sayed, G.O., Dessouki, H.A., Ibrahiem, S.S., 2011. Removal of Zn(II), Cd(II) and Mn(II) from aqueous solutions by adsorption on maize stalks. Malaysian J. Anal. Sci. 15, 8-21.

15. Farooq, U., Khan, M.A., Athar, M., Kozinski, J.A., 2011. Effect of modification of environmentally friendly biosorbent wheat (Triticum aestivum) on the biosorptive removal of cadmium(II) ions from aqueous solution. Chem. Eng. J. 171, 400-410.

16. Feng, N., Guo, X., Liang, S., Zhu, Y., Liu, J., 2011. Biosorption of heavy metals from aqueous solutions by chemically modified orange peel. J. Hazard. Mater. 185, 49-54.

17. Feng, N., Guo, X., 2012. Characterization of adsorptive capacity and mechanisms on adsorption of copper, lead and zinc by modified orange peel. Trans. Nonferrous Met. Soc. China 22, 1224-1231.

18. Flores-Garnica, J.G., Morales-Barrera, L., Pineda-Camacho, G., Cristiani-Urbina, E., 2013. Biosorption of Ni(II) from aqueous solutions by Litchi chinensis seeds. Bioresour. Technol. 136, 635-643.

19. Futalan,C.M., Tsai,W.C., Lin, S., Hsien, K., Dalida, M.L., Wan, M., 2012. Copper, nickel and lead adsorption from aqueous solution using chitosan-immobilized on bentonite in a ternary system. Sustain. Environ. Res. 22, 345-355.

20. Gala, A., Sanak-Rydlewska, S., 2011. A comparison of Pb(II) sorption from aqueous solutions on walnut shells and plum stones. Polish J. Environ. Stud. 20, 877-883.

21. García-Rosales, G., Colín-Cruz, A., 2010. Biosorption of lead by maize (Zea mays) stalk sponge. J. Environ. Manage. 91, 2079-2086.

22. García-Mendieta, A., Olguín, M.T., Solache-Ríos, M., 2012. Biosorption properties of green tomato husk (Physalis philadelphica Lam) for iron, manganese and iron-manganese from aqueous systems. Desalin. 284, 167-174.

23. Giri, A.K., Patel, R., Mandal, S., 2012. Removal of Cr(VI) from aqueous solution by Eichhornia crassipes root biomass-derived activated carbon. Chem. Eng. J. 185-186, 71-81.

24. Gonen, F., Serin, D.S., 2012. Adsorption study on orange peel: removal of Ni(II) ions from aqueous solution. Afr. J. Biotechnol. 11, 1250-1258.

25. Goyal, P., Srivastava, S., 2009. Characterization of novel Zea mays based biomaterial designed for toxic metals biosorption. J. Hazard. Mater. 172, 1206-1211.

26. Gutha, Y., Munagapati, V.S., Alla, S.R., Abburi, K., 2011. Biosorptive removal of Ni(II) from aqueous solution by Caesalpinia bonducella seed powder. Sep. Sci. Technol. 46, 2291-2297.

27. Horsfall, J.M., Abia, A.A., Spiff, A.I., 2006. Kinetic studies on the adsorption of Cd(II), Cu(II) and Zn(II) ions from aqueous solutions by Cassava (Munihot sculenta Cranz) tuber bark waste. Bioresour. Technol. 97, 283-291.

28. Hossain, M.A., Ngo, H.H., Guo, W.S., Setiadi, T., 2012a. Adsorption and desorption of copper(II) ions onto garden grass. Bioresour. Technol. 121, 386-395.

29. Hossain, M.A., Ngo, H.H., Guo, W.S., Nguyen, T.V., 2012b. Palm oil fruit shells as biosorbent for copper removal from water and wastewater: Experiments and sorption models. Bioresour. Technol.113, 97-101.

30. Hu, X., Zhao, M., Song, G., Huang, H., 2011. Modification of pineapple peel fiber with succinic anhydride for Cu(II), Cd(II) and Pb(II) removal from aqueous solutions. Environ. Technol. 32, 739-746.

31. Jiménez-Cedillo, M.J., Olguín, M.T., Fall, C., Colin-Cruz, A., 2013. As(III) and As(V) sorption on iron-modified non-pyrolyzed and pyrolyzed biomass from Petroselinum crispum (parsley). J. Environ. Manage. 117, 242-52.

32. Kannan, N., Veemaraj, T., 2010. Batch adsorption dynamics and equilibrium studies for the removal of Cd(II) ions from aqueous solution using jack fruit seed and commercial activated carbons - a comparative study. Electron. J. Environ. Agric. Food. Chem. 9, 327-336.

33. Kelly-Vargas, K., Cerro-Lopez, M., Reyna-Tellez, S., Bandala, E.R., Sanchez-Salas, J.L., 2012. Biosorption of heavy metals in polluted water, using different waste fruit cortex. Phys. Chem. Earth 37-39, 26-29.

34. Kumar, U., Bandyopadhyay, M., 2006. Sorption of cadmium from aqueous solution using pretreated rice husk. Bioresour. Technol. 97, 104-109.

35. Kumar, J., Balomajumder, C., Mondal, P., 2011. Application of agro-based biomasses for zinc removal from wastewater - a review. Clean - Soil Air Water 39, 641-652.

36. Kumar, P.S., Ramalingam, S., Sathyaselvabala, V., Kirupha, S.D., Murugesan, A., Sivanesan, S., 2012. Removal of Cd(II) from aqueous solution by agricultural waste cashew nut shell. Korean J. Chem. Eng. 29, 756-768.

37. Kurniawan, A. Kosasih, A.N., Febriano, J. Ju, Y.H., Sunarso, J. Indraswati, N., 2011. Evaluation of cassava peel waste as lowcost biosorbent for Ni sorption: equilibrium, kinetics, thermodynamics and mechanisms. Chem. Eng. J. 172, 158-166.

38. Lasheen, M.R., Ammar, N.S., Ibrahim, H.S., 2012. Adsorption/desorption of Cd(II), Cu(II) and Pb(II) using chemically modified orange peel: Equilibrium and kinetic studies. Solid State Sci. 14, 202-210.

39. Liang, S., Guo, X., Feng, N., Tian, Q., 2009. Adsorption of Cu(II) and Cd(II) from aqueous solution by mercapto-acetic acid modified orange peel. Colloids and Surfaces

B: Biointerfaces 73, 10-14.

40. Liu, C., Ngo, H.H., Guo, W.S., (2012) Watermelon Rind: Agro-waste or Superior Biosorbent. Appl. Biochem. Biotechnol. 167, 1699-1715.

41. Majumdar, S., Mandal, P.K., Das, S.K., Majumdar, R., 2013. Metal impregnated silica-carbon materials from rice husk: a versatile sorbent for toxic organics and inorganics in water and air. Clean - Soil Air Water 41, 291-297.

42. Manzoor, Q., Nadeem, R., Iqbal, M., Saeed, R., Ansari, T.M., 2013. Organic acids pretreatment effect on Rosa bourbonia phyto-biomass for removal of Pb(II) and Cu(II) from aqueous media. Bioresour. Technol. 132, 446-452.

43. Marin, A.B.P., Aguilar, M.I., Ortuno, J.F., Meseguer, V.F., Saez, J., Florenz, M., 2010. Biosorption of Zn(II) by orange waste in batch and packed bed systems. J. Chem. Technol. Biotechnol., 85, 1310-1318.

44. Marin-Rangel, V.M., Cortes-Martines, R., Villanueva, R.A.C., Garnica - Romo, M.G., Martinez-Flores, H.E., 2012. As(V) biosorption in an aqueous solution using chemically treated lemon (Citrus aurantifolia swingle) residues. J. Food Sci. 71, 10-14.

45. Mishra, V., Balomajumder, C., Agarwal, V.K., 2010. Zn(II) ion biosorption onto surface of Eucalyptus leaf biomass: isotherm, kinetic, and mechanistic modeling. Clean – Soil Air Water 38, 1062-1073.

46. Mishra, V., Balomajumder, C., Agarwal, V.K., 2012. Kinetics, mechanistic and thermodynamics of Zn(II) ion sorption: a modeling approach. Clean – Soil Air Water 40, 718-727.

47. Mosa, A.A., El-Ghamry, A., Trüby, P., 2011. Chemically modified crop residues as a low-cost technique for the removal of heavy metal ions from wastewater. Water Air Soil Pollut. 217, 637-647.

48. Muxel, A.A., Gimenez, S.M.N., Almeida, F.A.S., Alfaya, R.V.S., Alfaya, A.A.S., 2011. Cotton fiber/ZrO2, a new material for adsorption of Cr(VI) ions in water. Clean – Soil Air Water 39, 289-295.

49. Netzahuatl-Muñoz, A.R., Guillén-Jiménez, F.D.M., Chávez-Gómez, B., Villegas-Garrido, T.L., Cristiani-Urbina, E., 2012. Kinetic study of the effect of pH on hexavalent and trivalent chromium removal from aqueous solution by Cupressus lusitanica bark. Water Air Soil Pollut. 223, 625-641.

50. Nguyen, T.V., Vigneswaran, S., Ngo, H.H., Kandasamy, J., 2010. Arsenic removal by iron oxide coated sponge: experimental performance and mathermatical models. J. Hazard. Mater., 182, 723-729.

51. Njoku, V.O., Ayuk, A.A., Ejike, E.E., Oguzie, E.E., Duru, C.E., Bello, O.S., 2011. Cocoa pod husk as a low cost biosorbent for the removal of Pb(II) and Cu(II) from

aqueous solutions. Aust. J. Basic Appl. Sci. 5, 101-110.

52. Ofomaja, A.E., Naidoo, E.B., Modise, S.J., 2010. Biosorption of Cu(II) and Pb(II) onto potassium hydroxide treated pine cone powder. J. Environ. Manage. 91, 1674-1685.

53. Okoro, I.A., Okoro, S.O., 2011. Agricultural byproducts as green chemistry absorbents for the removal and recovery of metal ions from wastewater environment. Continental J. Water Air Soil Pollut. 2, 15-22.

54. Osman, H.E., Badwy, R.K., Ahmad, H.F., 2010. Usage of some agricultural by-products in the removal of some heavy metals from industrial wastewater. J. Phytol. 2, 51-62.

55. Panda, G.C., Das, S.K., Guha, A.K., 2008. Biosorption of cadmium and nickel by functionalized husk of Lathyrus sativus. Colloids and Surfaces B: Biointerfaces 62, 173-179.

56. Park, D., Yun, Y., Park, J.M., 2010. The past, present, and future trends of biosorption. Biotechnol. Bioprocess Eng. 15, 86-102.

57. Reddy, D.H.K., Harinath, Y. , Seshaiah, K., Reddy, A.V.R., 2010. Biosorption of lead from aqueous solutions using chemically modified Moringa oleifera tree leaves. Chem. Eng. J. 162, 626-634.

58. Reddy, D.H.K., Ramana, D.K.V., Seshaiah, K., Reddy, A.V.R., 2011. Biosorption of Ni(II) from aqueous phase by Moringa oleifera bark, a low cost biosorbent. Desalin. 268, 150-157.

59. Sahmoune, M.N., Louhab, K., Boukhiar, A., 2011. Advanced biosorbents materials for removal of chromium from water and wastewaters. Environ. Prog. Sustainable Energy 30, 284-293.

60. Saka, C., Sahin, O., Demir, H., Kahyaoglu, M., 2011. Removal of lead from aqueous solutions using preboiled and formaldehyde treated onion skins as a new adsorbent. Sep. Sci. Technol. 46, 507-517.

61. Saka, C., Sahin, O., Kucuk, M.M., 2012. Application on agricultural and forest waste adsorbents for the removal of Pb(II) from contaminated waters. Int. J. Environ. Sci. Technol. 9, 379-394.

62. Sen, T.K., Mohammod, M., Maitra, S., Dutta, B.K., 2010. Removal of cadmium from aqueous solution using castor seed hull: a kinetic and equilibrium study. Clean – Soil Air Water 38, 850-858.

63. Senthil Kumar, P., Ramalingam, S., Abhinaya, R.V., Kirupha, S.D., Murugesan, A., Sivanesan, S., 2012. Adsorption of metal ions onto the chemically modified agricultural waste. Clean - Soil Air Water 40, 188-197.

64. Shafique, U., Ijaz, A., Salman, M., Zaman, W., Jamil, N., Rehman, R., Javaid, A., 2012.

Removal of arsenic from water using pine leaves. J. Taiwan Inst. Chem. Eng. 43, 256-263.

65. Shafqat, F., Bhatt, H.N., Hanif, M.A., Zubair, A., 2008. Kinetic and equilibrium studies of Cr(III) and Cr(VI) sorption from aqueous solution using Rosa gruss a teplitz (red rose) waste biomass. J. Chil. Chem. Soc. 53, 1667-1672.

66. Shahmohammadi-Kalalagh, S., Babazadeh, H., Nazemi, A.H., Manshouri, M., 2011. Isotherm and kinetic studies on adsorption of Pb, Zn and Cu by kaolinite. Caspian J. Environ. Sci. 9, 243-255.

67. Suksabye, P. and Thiravetyan, P., 2012. Cr(VI) adsorption from electroplating plating wastewater by chemically modified coir pith. J. Environ. Manage. 102, 1-8.

68. Taha, G.M., Arifien, A.E., El-Nahas, S., 2011. Removal efficiency of potato peels as a new biosorbent material for uptake of Pb(II), Cd(II) and Zn(II) from the aqueous solutions. J. Solid Waste Technol. Manage. 37, 128-140.

69. Turan, N.G., Mesci, B., 2011. Use of pistachio shells as an adsorbent for the removal of zinc ion. Clean – Soil Air Water 39, 475-481.

70. Vázquez, G., Mosquera, O., Freire, M.S., Antorrena, G., González-álvarez, J., 2012. Alkaline pre-treatment of waste chestnut shell from a food industry to enhance cadmium, copper, lead and zinc ions removal. Chem. Eng. J. 184, 147-155.

71. Vimala, R., Das, N., 2009. Biosorption of Cd(II) and Pb(II) from aqueous solutions using mushrooms: A comparative study. J. Hazard. Mater. 168, 376-382.

72. Wan Ngah, W.S., Hanafiah, M.A.K.M., 2008. Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: A review. Bioresour. Technol. 99, 3935-3948.

73. Wang, J., Chen, C., 2010. Research advances in heavy metal removal by biosorption. Acta Sci. Circumstantiae 30, 673-701.

74. Witek-Krowiak, A., Harikishore, K., Reddy, D., 2013. Removal of microelemental Cr(III) and Cu(II) by using soybean meal waste: Unusual isotherms and insights of binding mechanism. Bioresour. Technol. 127, 350-357.

75. Zafar, M.N., Nadeem, R., Hanif, M.A., 2007. Biosorption of nickel from protonated rice bran. J. Hazard. Mater. 143, 478-485.

FIRGURE CAPTIONS 1

1

Figure 1 2

Mechanisms of heavy metal capture onto AWBs. 3

Figure 2 4

Modification methods for producing better AWBs. 5

6

1

2 3 Figure 14

Figure 2

34

1 TABLES 2

3

Table 1 The maximum contaminant levels (MCL) for the most common heavy metals 4 (adapted from Barakat, 2011). 5

Heavy metal Toxicities MCL b (mg/L) Arsenic (As) Skin manifestations, visceral cancers, vascular disease 0.05 Cadmium (Cd) Kidney damage, renal disorder, human carcinogen 0.01 Chromium (Cr) Headache, diarrhea, nausea, vomiting, carcinogenic 0.05 Copper (Cu) Liver damage, Wilson disease, insomnia 0.25 Nickel (Ni) Dermatitis, nausea, chronic asthma, coughing, human carcinogen 0.20 Zinc (Zn) Depression, lethargy, neurological signs and increased thirst 0.80 Lead (Pb) Damage the fetal brain, diseases of the kidneys, circulatory system, and nervous system 0.006 Mercury (Hg) Rheumatoid arthritis, and diseases of the kidneys, circulatory system, and nervous system 0.00003

6

35

1 Table 2 Comparison of different technologies for removing heavy metals from 2 wastewater 3

No Methods Disadvantages Advantages 1 Chemical precipitation Large amounts of sludge

Extra operational cost for sludge disposal Simple operation, inexpensive, can remove most of metals

2 Chemical coagulation High cost Large consumption of chemicals

Sludge settling Dewatering

3 Ion-exchange High cost Less number of metal ions removed

High regeneration of materials Metal selective

4 Electrochemical methods High capital and running cost Initial solution pH and current density

Metal selective No consumption of chemicals Pure metals can be achieved

5 Adsorption using activated carbon Cost of activated carbon No regeneration Performance depends upon adsorbent

Most of metals can be removed High efficiency (>99%)

6 Biosorption Early saturation, limited potential for biological process improvement, no potential for biologically altering the metal valence state

Low cost, high efficiency, minimization of sludge, regeneration of biosorbents, no additional nutrient requirement, metal recovery

7 Membrane filtration

High operational cost due to membrane fouling

Small space requirement, low pressure, high separation selectivity

8 Electro dialysis High operational cost due to membrane fouling and energy consumption

High separation selectivity

9 Photo catalysis

Long duration time, limited applications

Removal of metals and organic pollutant simultaneously, less harmful by-products

4

5

6

36

Table 3 Comparison of AWBs with conventional adsorbents using the maximum 1 adsorption capacity (qmax - mg/g). 2 3 Metal ion Adsorbent Type of adsorbent qmax (mg/g) Reference As(V) Parsley MB (Iron modified non-pyrolyzed) 0.19 Jiménez-Cedillo et al., 2013 Hematite CA 0.20 Shafique et al., 2012 Kaolinite CA 0.23 Shafique et al., 2012 Lemon residues MB (FeCl3) 0.47 Marin-Rangel et al., 2012 Pine leaves NB 3.27 Shafique et al., 2012 Sponge CA (Iron oxide coated) 4.50 Nguyen et al., 2010 Coconut coir pith MB (Hydrochloric acid,

epichlorohydrin, dimethylamine ) 13.75 Anirudhan and Unnithan, 2007

Parsley MB (Iron modified pyrolyzed) 18.17 Jiménez-Cedillo et al., 2013 Cd(II) Commercial activated

carbon CA 0.70 Kannan and Veemaraj, 2010

Granular activated carbon CA 1.39 Sen et al., 2010 Castor seed hull NB 6.98 Sen et al., 2010 Commercial activated

carbon F.400 CA 8.21 Azouaou et al., 2010

Coffee grounds NB 15.65 Azouaou et al., 2010 Maize stalks NB 18.05 El-Sayed et al., 2011 Rice husk MB (NaOH) 20.24 Kumar and Bandyopadhyay,

2006 Cashew nut shell NB 22.11 Kumar et al., 2012 Pineapple peel fibre MB (Succinic anhydride) 34.18 Hu et al., 2011 Wheat straw MB (Urea) 39.22 Farooq et al., 2011 Husk of Lathyrus sativus MB (Introducing thio groups) 52.80 Panda et al., 2008 Cortex banana waste NB 67.20 Kelly-Vargas et al., 2012 Orange peel MB (Mercapto-acetic acid) 136.05 Liang et al., 2009 Orange peel MB (grafted copolymerization) 293.3 Feng et al., 2011 Cashew nut shell MB (H2SO4) 436.7 Senthil Kumar et al., 2012 Cr(VI) Water lily flower NB 8.44 Elangovan et al., 2008 Mangrove leaves NB 8.87 Elangovan et al., 2008 Eichhornia crassipes root

activated carbon NB 36.34 Giri et al., 2012

Red rose distillation waste MB (Sodium alginate) 57.68 Shafqat et al., 2008 Activated carbon

(Filtrasorb-400) CA 57.70 Arulkumar et al., 2012

Cotton fiber MB (ZrO2 composite) 69.15 Muxel et al., 2011 Cross-link chitosan CA 78.00 Suksabye and Thiravetyan, 2012 Cupressus lusitanica bark 87.5 Netzahuatl-Muñoz et al., 2012 Prawn shell activated carbon NB 100.60 Arulkumar et al., 2012 Durian shell waste NB 117.00 Kurniawan et al., 2011 Commercial activated

carbon CA 153.96 Suksabye and Thiravetyan, 2012

Coir pith NB 165.00 Suksabye and Thiravetyan, 2012 Coir pith MB (Grafted with acrylic acid) 196.00 Suksabye and Thiravetyan, 2012 Cu(II) Bentonite CA 4.00 Aman et al., 2008 Kaolinite CA 4.42 Shahmohammadi-Kalalagh et

al., 2011 Zeolite CA 5.20 Aman et al., 2008 Chitosan immobilized on

bentonite CA 12.60 Futalan et al., 2012

Pine cone powder MB (KOH) 26.32 Ofomaja et al., 2010 Ion exchange resins CA 26.73 Banerjee et al., 2012 Pineapple peel fibre MB (Succinic anhydride) 27.68 Hu et al., 2011 Cortex banana waste NB 36.00 Kelly-Vargas et al., 2012 Garden grass NB 58.34 Hossain et al., 2012a Palm oil fruit shell NB 60.00 Hossain et al., 2012b Cortex orange waste NB 67.20 Kelly-Vargas et al., 2012 Cortex lemon waste NB 70.40 Kelly-Vargas et al., 2012 Orange peel MB (Mercapto-acetic acid) 70.67 Liang et al., 2009 Orange peel MB (NaOH and CaCl2) 70.73 Feng and Guo, 2012 Tamarin seed NB 82.97 Chowdhury and Saha, 2011 Watermelon shell NB 111.10 Banerjee et al., 2012 Rose petals waste NB 124.21 Manzoor et al., 2013 Cashew nut shell MB (H2SO4) 406.6 Senthil Kumar et al., 2012 Ni(II) Chitosan immobilized on CA 6.10 Futalan et al., 2012

37

bentonite Moringa oleifera bark NB 30. 38 Reddy et al., 2011 Powder activated carbon CA 31.08 Gutha et al., 2011 Rice bran MB (H3PO4) 46.51 Zafar et al., 2007 Cassava peel NB 57.00 Kurniawan et al., 2011 Orange peel NB 62.30 Gonen and Serin, 2012 Litchi chinensis seeds NB 66.62 Flores-Garnica et al., 2013 Orange peel MB (grafted copolymerization) 162.60 Feng et al., 2011 Caesalpinia bonducella seed MB (NaOH, H2SO4) 188.67 Gutha et al., 2011 Cashew nut shell MB (H2SO4) 456.30 Senthil Kumar et al., 2012 Pb(II) Commercial activated

carbon CA 5.90 Dubey and Shiwani, 2012

Kaolinite CA 7.75 Shahmohammadi-Kalalagh et al., 2011

Chitosan immobilized on bentonite

CA 15.00 Futalan et al., 2012

Portulaca plant biomass NB 17.24 Dubey and Shiwani, 2012 Cocoa pod husk NB 20.10 Njoku et al., 2011 Pine cone powder MB (KOH) 32.26 Ofomaja et al., 2010 Pineapple peel fibre MB (Succinic anhydride) 70.29 Hu et al., 2011 Cortex orange waste NB 76.80 Kelly-Vargas et al., 2012 Watermelon rind NB 98.06 Liu et al., 2012 Red rose waste MB (Methanol) 99.72 Bhatti et al., 2011 Rose petals waste NB 119.92 Manzoor et al., 2013 Onion skins MB (Boiled+Formaldehyde) 200.00 Saka et al., 2011 Orange peel MB (NaOH and CaCl2) 209.8 Feng and Guo, 2012 Orange peel MB (grafted copolymerization) 476.1 Feng et al., 2011 Zn(II) Bentonite clay CA (calcined) 4.95 Araujo et al., 2013 Kaolinite CA 4.95 Shahmohammadi-Kalalagh et

al., 2011 Maize stalks NB 30.30 El-Sayed et al., 2011 Orange waste NB 43.16 Marin et al., 2010 Orange peel MB (NaOH and CaCl2) 56.18 Feng and Guo, 2012 Cassava tuber bark waste MB (Thioglycollic acid) 83.30 Horsfall et al., 2006 Cedrus deodara sawdust NB 97.39 Mishra et al., 2012 Cashew nut shell MB (H2SO4) 455.7 Senthil Kumar et al., 2012 Note: CA: Conventional adsorbent; NB: Natural or unmodified biosorbent; MB:Modified biosorbent. 1 2 3

38

Table 4 Optimum pH values in various heavy metals - AWBs adsorption systems. 1

Metal ion Adsorbent Optimum pH Reference As(V) Pine leaves 4.0 Shafique et al., 2012 Cd(II) Coffee grounds 7.0 Azouaou et al., 2010 Maize stalks 6.0 El-Sayed et al., 2011 Wheat straw 6.0 Farooq et al., 2011 Pineapple peel fibre 7.5 Hu et al., 2011 Cashew nut shell 5.0 Kumar et al., 2012 Husk of Lathyrus sativus 5.0 Panda et al., 2008 Castor seed hull 6.0 Sen et al., 2010 Cashew nut shell 5.0 Senthil Kumar et al., 2012 Potato peels 6.0 Taha et al., 2011 Oyster mushroom, button mushroom

and milky mushroom 6.0 Vimala and Das, 2009

Cr(VI) Eichhornia crassipes root activated

carbon 4.5 Giri et al., 2012

Durian shell waste 2.5 Kurniawan et al., 2011 Cotton fiber 4.0 Muxel et al., 2011 Coir pith 2.0 Suksabye and Thiravetyan, 2012 Cu(II) Potato peels 6.0 Aman et al., 2008 Watermelon shell 8.0 Banerjee et al., 2012 Garden grass 6.0 Hossain et al., 2012a Palm oil fruit shell 6.5 Hossain et al., 2012b Pineapple peel fibre 5.4 Hu et al., 2011 Orange peel 5.0-7.0 Liang et al., 2009 Watermelon rind 5.0 Liu et al., 2012 Cocoa pod husk 6.0 Njoku et al., 2011 Pine cone powder 5.0 Ofomaja et al., 2010 Cashew nut shell 5.0 Senthil Kumar et al., 2012 Soybean meal waste 5.0 Witek-Krowiak et al., 2013 Ni(II) Litchi chinensis seeds 7.5 Flores-Garnica et al., 2013 Orange peel 5.0 Gonen and Serin, 2012 Caesalpinia bonducella seed 5.0 Gutha et al., 2011 Cassava peel 4.5 Kurniawan et al., 2011 Moringa oleifera bark 6.0 Reddy et al., 2011 Cashew nut shell 5.0 Senthil Kumar et al., 2012 Rice bran 6.0 Zafar et al., 2007 Pb(II) Portulaca plant biomass 6.0 Dubey and Shiwani, 2012 Pineapple peel fibre 5.6 Hu et al., 2011 Watermelon rind 6.8 Liu et al., 2012 Cocoa pod husk 6.0 Njoku et al., 2011 Pine cone powder 5.0 Ofomaja et al., 2010 Onion skins 6.0 Saka et al., 2011 Potato peels 6.0 Taha et al., 2011 Oyster mushroom, button mushroom

and milky mushroom 5.0 Vimala and Das, 2009

Zn(II) Citrus reticulata (Kinnow) waste 6.0 Boota et al., 2009 Maize stalks 5.0 El-Sayed et al., 2011 Watermelon rind 6.8 Liu et al., 2012 Orange waste 6.0 Marin et al., 2010 Eucalyptus leaf biomass 5.0 Mishra et al., 2010 Cedrus deodara sawdust 5.0 Mishra et al., 2012 Cashew nut shell 5.0 Senthil Kumar et al., 2012 Potato peels 6.0 Taha et al., 2011 2

39

Table 5 Effects of modifications of AWBs on heavy metal removal from wastewater. 1 Biosorbents Modifying agents Metal ions Changes in adsorption

capacity (%) References

Wheat straw Urea Cd(II) ↑ 822.82 Farooq et al., 2011 Orange peel NaOH 0.8M and

CaCl2 0.8M Cu(II)

↑ 59.73

Feng and Guo, 2012

Orange peel NaOH 0.8M and CaCl2 0.8M

Pb(II)

↑84.84

Feng and Guo, 2012

Orange peel NaOH 0.8M and CaCl2 0.8M

Zn(II) ↑ 164.38 Feng and Guo, 2012

Orange peel The grafted polymerization Pb(II)

↑ 420

Feng et al., 2011

Orange peel The grafted polymerization Cd(II)

↑ 460

Feng et al., 2011

Orange peel The grafted polymerization Ni(II) ↑ 1650 Feng et al., 2011 Green tomato husk Formaldehyde 0.2% Fe(III) ↑ 5.09 García-Mendieta et al. ,

2012 Green tomato husk Formaldehyde 0.2% Mn(II) ↑ 10.89 García-Mendieta et al. ,

2012 Zea mays cob powder Acetylation Cr(III) ↑2-8 Goyal and Srivastava,

2009 Pineapple peel fibber Succinic anhydride Cu(II)

↑336.26

Hu et al., 2011

Pineapple peel fibber Succinic anhydride Cd(II)

↑ 374.05

Hu et al., 2011

Pineapple peel fibber Succinic anhydride Pb(II) ↑ 242.7 Hu et al., 2011 Parsley Pyrolysis + FeCl3 As(V) ↑ 9463.16 Jiménez-Cedillo et al.,

2013 Parsley Pyrolysis + FeCl3 As(III) ↑ 416.67

Jiménez-Cedillo et al., 2013

Rice husk Epichlorohydrin, NaOH, NaHCO3

Cd(II) ↑ 29.60, 135.90, 88.58

Kumar and Bandyopadhyay, 2006

Corncobs Thermal treatment (1800C); H3PO4

Zn(II) ↑93.96 Kumar et al., 2011

Orange peel HNO3 0.1M Cd(II) ↑ 61.38 Lasheen et al., 2012 Orange peel Mercapto acetic acid Cu(II) ↑ 38.73 Liang et al., 2009 Orange peel Mercapto acetic acid Cd(II) ↑ 185.82 Liang et al., 2009 Rosa bournobia phyto biomass

Acetic acid, benzoic acid, citric acid 0.1N

Pb(II) ↓ 39.64, 43.89, 23.01 Manzoor et al., 2013

Rosa bournobia phyto biomass

Acetic acid, benzoic acid, citric acid 0.1N

Cu(II) ↓ 14.10, 15.19, 20.24

Manzoor et al., 2013

Husk of Lathyrus sativus Introducing thio groups Cd(II)

↑50

Panda et al., 2008

Rosa gruss an teplitz (red rose) distillation waste

Sodium alginate Cr(III)

↑ 23.36

Shafqat et al., 2008

Rosa gruss an teplitz (red rose) distillation waste

Sodium alginate Cr(VI) ↑ 18.31 Shafqat et al., 2008

2

3

4

5 6

Highlights

Ability of AWBs for heavy metal detoxification was evaluated.

Influential factors on heavy metal biosorption were presented.

Insights of binding mechanism were revealed and essential tools were introduced.

Merits and demerits of pretreatment methods for better biosorbents were highlighted.

Recommendation to use AWBs as green and cost-effective biosorbents was made.