Assessment of soil washing for Zn contaminated soils using various washing solutions
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Journal of Industrial and Engineering Chemistry 18 (2012) 822825
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jou r n al h o mep ag e: w ww .e1. Introduction
In the Republic of Korea, Zn contamination in soils has receivedgreat attention lately because of its toxicity and its prevalence atnumerous contaminated industrial areas. Zn is listed as a prioritypollutant by the United States Environmental Protection Agency(USEPA) . Several remediation techniques for heavy metalcontaminated soils have been described  including soilwashing which can extract heavy metals or metalloids that areadsorbed or precipitated onto the surface of the soils particles. Inthe Republic of Korea, aqua regia extraction is used to evaluate theeffectiveness of the remediation process for Zn contaminated soils. Therefore, among various remediation techniques soil washingis a viable remedial process option to remove Zn from soils eventhough waste water treatment is required post soil washing.Selection of the right washing solution is the most important factorin determining the extraction effectiveness. The selection of thetype of washing solution depends on the target contaminants, thebonding/chelating strength of the extraction solution, and the soilcharacteristics . Numerous researchers have investigated
different types of washing solutions (e.g., inorganic salts,inorganic acids, organic acids, and alkaline agents) and thesolutions heavy metal and metalloid removal effectiveness fromsoils. For example, Ko et al.  reported that a 67% Zn removal ratewas achieved in a washing scrubber from a control sample(335 mg/kg) after 25 min using HCl with a liquid to solid ratio of10 to 1. Even though strong acids are expected to be more effectivefor soil washing, destruction of the soil fabric caused by theapplication of strong acids encouraged researchers to test weakacids with metal chelating capacities. For example, Wuana et al. tested citric acid, tartaric acid, and EDTA for the removal of Ni,Cu, Zn, Cd, and Pb, from articially contaminated soils, with mixedresults. Similarly, Elliott and Shastri  tested oxalate salt tochelate Cd, Pb, and Zn from heavily contaminated soils, also withmixed results where exchangeable metals were more easilyleached out. However, a systematic study to assess soil washingsolutions to remove Zn from contaminated soils is lacking. In thisstudy, seven different washing solutions encompassing strongacids, weak acids, and strong bases, were tested to remove Zn incontaminated soil.
The purpose of this study was to select the most effective andcost saving soil washing solution. The effectiveness of the washingprocess was evaluated by measuring the residual Zn concentra-tions after the washing process. The residual Zn concentrationswere compared to the Korean warning standard of 600 mg/kg and300 mg/kg for forest land and residential areas, respectively.
solid ratio of 10. The soil washing results showed the following order of washing solution decreasing
effectiveness for the removal of Zn: HCl > HNO3 > H2SO4 > H3PO4 > C4H6O6 > HOOCCOOH2H2O > NaOH. 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
* Corresponding author at: Department of Environmental Engineering, Chonnam
National University, Gwangju 500-757, Republic of Korea. Tel.: +82 62 530 1855;
fax: +82 62 530 0856.
E-mail address: email@example.com (J.-H. Park).
1226-086X/$ see front matter 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.doi:10.1016/j.jiec.2011.11.137Assessment of soil washing for Zn contawashing solutions
Deok Hyun Moon a, Ju-Ry Lee b, Mahmoud Wazne c
aDepartment of Environmental Engineering, Chosun University, Gwangju 501-759, RepubDepartment of Environmental Engineering, Chonnam National University, Gwangju 500cDepartment of Environmental Engineering, Stevens Institute of Technology, Hoboken, Nd Soil Technology Research Institute, Chonnam National University, Gwangju 500-757, Re
A R T I C L E I N F O
Received 16 July 2011
Accepted 12 November 2011
Available online 21 December 2011
A B S T R A C T
Bench-scale soil washing ex
washing solutions including
acid (HOOCCOOH2H2O), sulused. The concentration of thinated soils using various
eong-Hun Park b,d,*
c of Korea
57, Republic of Korea
blic of Korea
riments were conducted to remove Zn from contaminated soils. Various
drochloric acid (HCl), nitric acid (HNO3), sodium hydroxide (NaOH), oxalic
ric acid (H2SO4), phosphoric acid (H3PO4), and tartaric acid (C4H6O6) were
ashing solutions used in this study ranged from 0.1 M to 2 M with a liquid to
l sev ier . co m / loc ate / j iec
Therefore, DI water is not a viable solution to breaking up of thestrong bonds that Zn may have with the mineral surface.
The washing results using seven different washing solutionsshowed that HCl was the most effective solution for the removal ofZn from the contaminated soils. The residual Zn concentrations in
Table 1Major oxide chemistry for the Zn contaminated soil.
Chemical properties Soil (wt%)
Fig. 1. Zn concentrations remained in soil after hydrochloric acid (HCl) washing.
Fig. 2. Zn concentrations remained in soil after nitric acid (HNO3) washing.
Fig. 3. Zn concentrations remained in soil after sulfuric acid (H2SO4) washing.
D.H. Moon et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 822825 8232. Experimental methodology
2.1. Zn contaminated soils
Zn contaminated soils were collected from a steel company inChangwon, Gyeongsangnam-do, Republic of Korea. The soil pH wasmeasured in accordance with ASTM D 4980-89 . The contami-nated soil was classied using a particle size analysis system(Sedigraph 5100, USA). The bulk chemistry of Zn contaminated soilwas analyzed using X-ray uorescence (XRF). The total organiccontent (TOC) was obtained using the TOC-SSM-5000A (Shimadzu,Kyoto, Japan) analyzer. The Zn contaminated soil was air-dried andsieved using the #10 mesh (2 mm) in order to remove largeparticles and obtain a homogeneous soil size. The total Znconcentration in the soil was obtained by total digestion analysisafter sieving (#100 mesh) using a 3:1 HCl/HNO3 solution (aquaregia) .
2.2. Soil washing process
Reagent grade hydrochloric acid (HCl), nitric acid (HNO3),sodium hydroxide (NaOH), oxalic acid (HOOCCOOH2H2O, OA),sulfuric acid (H2SO4, SA), phosphoric acid (H3PO4, PA) and tartaricacid (C4H6O6, TA) were used as extraction agents. Theses washingsolutions are widely used for the removal of heavy metals andmetalloids [6,10,11]. The concentration of the washing solutionswas varied from 0.1 M to 2 M. The washing process was performedwith 5 g of soil mixed with 50 mL of washing solution in a 125 mLplastic bottle. The suspensions were then shaken at 200 RPM for1 h at 20 8C in a shaking incubator (LabTech, Daihan, South Korea).Additional experiments were run using 3 M acids or 2 h mixingtime for the most effective washing solution. Following the shakingprocess, the suspended solids were separated by ltration with a0.45-mm micropore lter and air-dried. After the washing process,the Zn concentration in the soil was measured based on the KoreanStandard Test methods (aqua regia) and compared to the Koreanwarning standards for a forest land and residential area . Thesoluble Zn concentrations were analyzed using an inductivelycoupled plasma-optical emission spectrometer (ICP-OES, Optima7000DV) (PerkinElmer, CT, USA). All sample analyses wereconducted in triplicate and averaged values were reported. Twoquality control standards and matrix spikes were used to validatethe accuracy and performance of the equipment.
3. Results and discussion
3.1. Characterization of Zn contaminated soil
The bulk chemistry of Zn contaminated soil was provided inTable 1. The soil pH value was measured at 8.31 and thecontaminated soil was classied as sandy soil. The soil wascomposed of 56% sand, 34% silt and 10% clay. The organic contentwas determined at 0.6% and the total Zn concentration wasmeasured at 4973 mg/kg.
3.2. Soil washing
The soil washing results are presented in Figs. 17. The soilwashing process using DI water only was not very effective atremoving Zn and the results showed a high residual Znconcentration of 2152.9 mg/kg (56.7% removal) post the washingprocess (Fig. 1). This indicated that Zn was not very soluble and Znextraction by DI water was limited. It has been reported thatinorganic contaminants can be incorporated into the recalcitrantmineral lattice or form strong bonds with the mineral surface .
D.H. Moon et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 822825824Fig. 4. Zn concentrations remained in soil after phosphoric acid (H3PO4) washing.the soil decreased with increasing HCl concentrations. Thesendings are similar to the ones published by Moutsatsou et al. ,where they reported that HCl was found more effective than H2SO4and HNO3 for the extraction of As, Cu, Pb, and Zn from soils
Fig. 5. Zn concentrations remained in soil after tartaric acid (C4H6O6) washing.
Fig. 6. Zn concentrations remained in soil after oxalic acid (HOOCCOOH2H2O)washing.
Fig. 7. Zn concentrations remained in soil after sodium hydroxide (NaOH) washing.contaminated by metallurgical materials In this study, the residualZn concentrations in the soil after washing were less than theKorean warning standard of 600 mg/kg for a forest land when theHCl concentration was greater than or equal to 0.5 M. The lowestZn concentration of 380 mg/kg (approximately 92.4% removal) wasattained with 2 M HCl. However, this Zn concentration was greaterthan the Korean warning standard of 300 m/kg for a residentialarea. A residual Zn concentration of 225 mg/kg (95% removal) wasattained after washing with 3 M HCl extraction solution. Therefore,in order to pass the 300 mg/kg warning standard, a higher HClconcentration (>3 M) was required. There was no major differencebetween washing periods of 1 h and 2 h with 2 M HCl, showingsimilar Zn concentrations of 380 mg/kg and 383 mg/kg, respec-tively. Moutsatsou et al.  reported that 1 h mixing time isusually sufcient for extraction of pollutants. They also showedthat mobilization of Zn was not improved by increasing mixingtime from 1 h to 2 h using 1 M HCl extraction solution with a solid/liquid = 30 g/L and a mixing speed of 150 RPM.
Moreover, the change in liquid to solid ratio from 10:1 to 20:1did not result in an increase of the extraction efciency, actuallyshowing a slightly higher Zn concentration of 435 mg/kg. It isreported that the main mechanisms of metal removal with HCl aredesorption enhanced by low pH, dissolution of discrete metalphases and specic soil components containing metals . It hasalso been reported that the mobilization rate of Zn in HCl solutionsremained constant going from 1 M to 3 M (67%) but it was greatlyincreased to 97% at 6 M HCl . However, in this study, a Znremoval of approximately 95% was attained with 3 M HCl. Thismay be due to the type of Zn compounds existing in the soilsamples used in this study. Sphalerite (ZnS) which is an insolubleZn precipitate was the main phase identied in the contaminatedsoil reported by Moutsatsou et al.  whereas this precipitate wasnot observed as a major phase using XRD in a previous study for thesame soil samples . The second effective washing solution forthe removal of Zn was HNO3. Similar to HCl, residual Znconcentrations decreased with increasing HNO3 concentrations.The lowest Zn concentration was observed at 404.4 mg/kg with a1 h washing period at 2 M HNO3. The third effective washingsolution for the removal of Zn was SA. Similar to the HCl soilwashing experiments, the residual Zn concentrations decreasedwith increasing SA concentrations. The lowest Zn concentrationwas measured at 473 mg/kg with a 1 h washing period and 2 M SA.The fourth effective washing solution was PA. The lowest residualZn concentration of 506.4 mg/kg was obtained with a 2 h washingperiod at 2 M PA. The rest of the washing solutions such as NaOH,OA and TA were not very effective, showing Zn concentrations ofmore than 850 mg/kg remaining after the washing process. Thismay indicate that most of the Zn is not readily exchangeableespecially as evidenced by the weak acids (OA and TA) experiments[7,8]. The least effective washing solution was NaOH, showing ahigh Zn concentration of 1634 mg/kg after washing with 2 M NaOH(67% removal). Similar extraction results were reported when Znfrom mine tailings was extracted using 0.12 M of NaOH .However, the Zn removal rate increased with the increase in NaOHconcentration. This may be due to the speciation of Zn in alkalineconditions. It has been reported that in the ZnH2O system,Zn(OH)3
and Zn(OH)42 are the prevailing Zn species in a high pH
aqueous environment .Overall, the Zn removal effectiveness sequence was in the
following order: HCl > HNO3 > SA > PA > TA > OA > NaOH.
Soil washing experiments were performed to remove Zn fromcontaminated soils. Seven different washing solutions includinghydrochloric acid (HCl), nitric acid (HNO3), sodium hydroxide
(NaOH), oxalic acid (HOOCCOOH2H2O), sulfuric acid (H2SO4),phosphoric acid (H3PO4), and tartaric acid (C4H6O6) were used withconcentrations ranging from 0.1 M to 3 M. The soil washing resultsshowed that the Zn removal effectiveness sequence was in thefollowing order: HCl > HNO3 > SA > PA > TA > OA > NaOH. Aconcentration greater than the 0.5 M HCl, HNO3 and H2SO4washing solutions was required in order to pass the Koreanwarning standard of 600 mg/kg for a forest land. Moreover, 3 MHCl was necessary to pass the Korean warning standard of 300 mg/kg for a residential area.
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