Application of Electrocoagulation and Electrolysis on the Precipitation of Heavy Metals and Particulate Solids in Washwater from the Soil Washing

Soil washing, ex situ mechanical technique, is one of the few permanent treatment alternatives to remove metal contaminants from soils by employing physical separation based on mineral processing technologies to remove discrete particles or metal-bearing particles and/or chemical extraction based on leaching or dissolving process to extract the metals from the soils into an aqueous solution. However, washwater remained from soil washing process contains discrete particulate particles along with heavy metals as solution phase to be treated separately, as well as this process can produce large amount of sludge that requires further treatment, slow metal precipitation, poor settling, the aggregation of metal precipitates. Electrical treatments including electrocoagulation and electrolysis can be effective in removing these substances from washwater. This paper reviews the theoretical models in applying electrocoagulation and electrolysis to remove heavy metals and discrete particulate particles in washwater by examining and comparing the status of washwater treatment technologies which have been undertaken, mostly in the US and EU for the period 1990-2012.

Surfactant-Enhanced Washing of Soils Contaminated with Wasted-Automotive Oils and the Quality of the Produced Wastewater

An old automotive industrial site located at Mexico City with many years of operation and contaminated with heavy oil hydrocarbons, particularly spent oils, was assessed for restoration using the surfactant enhanced soil washing (SESW) process. The main goal of this study was to characterize the contaminated soil in terms of TPHs, BTEX, PAHs, and metals contents as well as microbiologically (total heterotrophs and specific degrading microorganisms). We also aimed to determine the surfactant type and concentration to be used in the SESW process for the automotive waste oil contaminated soil. At the end, sixteen kg of contaminated soil were washed and the produced wastewater (approximately 40 L) was characterized in terms of COD, BOD;solids, and other physico-chemical parameters. The soil contained about 14,000 mg of TPH/kg soil (heavy fraction), 0.13 mg/kg of benzo (k) fluoranthene and 0.07 mg/kg of benzo (a) pyrene as well as traces of some metals. Metals concentrations were always under the maximum concentration levels suggested by Mexican regulations. 15 different surfactants were used to identify the one with the capability to achieve the highest TPH removal. Surfactants included 5 anionics, 2 zwitterionic, 5 nonionics and 3 natural gums. Sulfopon 30 at a concentration of 0.5% offered the best surfactant performance. The TPH removals employing the different surfactants were in the range from 38% to 68%, in comparison to the soil washing with water (10% of TPH removal). Once the surfactant was selected, 70 kg of soil were washed and the resulting water contained approximately 1300 mg/L of COD, 385 mg/L of BOD (BOD/COD = 0.29), 122 mg/L of MBAS, and 212 mg/L of oil and greases, among other contaminants.

An Empirical Framework of Washing Salinity by Irrigation to Maintain Soil Quality in Hetao Irrigation District

[Objectives] To summarize the characteristics of washing salinity by irrigation in Hetao Irrigation District, and propose the empirical framework of washing salinity by irrigation to maintain soil quality, and provide a theoretical basis for maintaining the sustainable development of soil in Hetao Irrigation District. [Methods] The methods of experiment, questionnaire, on-the-spot investigation and literature review were used. [Results] This study proposed the empirical framework of washing salinity by irrigation to maintain soil quality in Hetao Irrigation District. Seven factors of the framework, including flood irrigation, land leveling, plastic film mulching, fertilization, soil organic matter, pH and salinity, and their relationships were determined. The characteristics of these factors in Hetao Irrigation District were investigated(flooding irrigation with a large amount of irrigation water, high amount of fertilizer application, low organic matter, high pH, large variation of salinity, etc.). The mechanisms and effects of various factors affecting soil quality in Hetao Irrigation District were analyzed(the mean soil organic matter(SOM) and pH were kept in the range of 10.9-13.9 g/kg and 8.0-8.15 in recent 35 years, respectively, and increased slightly, etc.). [Conclusions] The empirical framework can be used as a theoretical norm for evaluating soil quality under the condition of washing salinity by irrigation. Under the condition of washing salinity by irrigation, the agricultural soil quality in Hetao Irrigation District showed a stable trend over time. Using this framework, we can find soil problems, and adjust some unbalanced factors to maintain the stability of soil quality in Hetao Irrigation District, and can also provide a reference for other areas.

Core-shell design of UiO66-Fe_(3)O_(4) configured with EDTA-assisted washing for rapid adsorption and simple recovery of heavy metal pollutants from soil

The coupling of washing with adsorption process can be adopted for the treatment of soils contaminated with heavy metals pollution.However,the complex environment of soil and the competitive behavior of leaching chemicals considerably restrain adsorption capacity of adsorbent material during washing process,which demands a higher resistance of the adsorbents to interference.In this study,we synthesized strongly magnetic,high specific surface area(573.49 m^(2)/g)UiO66 composites(i.e.,UiO66-Fe_(3)O_(4))using hydrothermal process.The UiO66-Fe_(3)O_(4) was applied as an adsorbent during the ethylene diamine tetraacetic acid(EDTA)-assisted washing process of contaminated soil.The incorporation of UiO66-Fe_(3)O_(4)results in rapid heavy metal removal and recovery from the soil under low concentrations of washing agent(0.001 mol/L)with reduced residual heavy metal mobility of soil after remediation.Furthermore,UiO66-Fe_(3)O_(4)can quickly recollect by an external magnet,which offers a simple and inexpensive recovery method for heavy metals from contaminated soil.Overall,UiO66-Fe_(3)O_(4)configuration with EDTA-assisted washing process showed opportunities for heavy metals contaminated sites.

Study of polluted soil remediation based on freezing and thawing cycles

It is generally known that soil pollution poses a terrible hazard to the environment, but the present techniques of contaminated soil remediation cannot control this growing threat. This paper compares the pollutant extraction efficiency of traditional pumping and treating, which is a typical washing technology for the remediation of contaminated soils, with methods that utilize freeze-thaw cycles. In the soil freezing process, water shifts from unfrozen soils to the freezing front, and the permeability of soil will be enhanced under certain temperature gradients and water conditions. Therefore, this paper discusses the purification of contaminated soil through freeze-thaw action. We conducted a cleansing experiment on clay and silica sand infused with NaCl（simulation of heavy metals） and found that the efficiency of purification was enhanced remarkably in the latter by the freeze-thaw action. To assess the effective extraction of DNAPLs in soil, we conducted an experiment on suction by freezing, predicated on the different freezing points of moisture and pollutants. We found that the permeability coefficient was significantly increased by the freezing-thawing action, enabling the DNAPL contaminants to be extracted selectively and effectively.

Facultative-anaerobic microbial digestion of coal preparation waste and use of effluent solids to enhance plant growth in a sandy soil

Coal preparation solid waste,which is a major environmental issue for coal-producing areas in China,may be microbiologically digested and transformed into a product suitable as a soil amendment to increase soil organic matter content and prevent and enhance plant/crop growth.Coal preparation waste collected from a coal sorting plant in Inner Mongolia,China was digested in bioreactors inoculated with microbial enrichments prepared from activated sludge and cow manure.The effluent solids from the coal preparation waste bioreactors were analyzed for their suitability as organic soil amendments,which complied with China standards.Plant growth tests were conducted in sandy soil from a semi-arid region in Colorado,which was amended with the effluent solids.Kentucky bluegrass(Poa pratensis L.)and chives(Allium schoenoprasum)were used as the representative plants for the growth tests,where results indicated substantially higher yields of Kentucky bluegrass and chives for the sandy soils amended with the effluent solids when compared to a commercial organic fertilizer.The number and average length of Kentucky bluegrass shoots were 10 and 5.1 times higher,respectively,in soils amended with the effluent solids.Similarly,the number and average length of chives shoots were 10 and 1.7 times higher,respectively,in soils amended with the effluent solids.Overall,the microbial digestion of coal preparation waste for application as an organic soil amendment is a viable alternative and beneficial use of coal preparation solid waste.

Extraction of Cd and Pb from contaminated-paddy soil with EDTA, DTPA, citric acid and FeCl3 and effects on soil fertility

Potentially toxic metals,Cd and Pb in paddy soil,have important meanings for safety of rice.A comparison extraction of Cd and Pb with EDTA,DTPA,citric acid,and FeCl3 and effects on soil fertility was studied.Results indicate that about 59%and 63%of soil Cd and Pb were simultaneously removed by 10 g/L EDTA at pH 5 with a soil/extractant ratio of 1:10(W/V)for 30 min while 52%and 51%by 5 g/L DTPA.Acid extractable and reducible Cd by EDTA and DTPA contributed 58%and 53%of the removals and acid extractable and reducible Pb were about 49%and 41%,respectively.Slight changes of soil fertility,including pH,cation exchange capacity,organic matter,and soil extractable phosphorus,were observed.Extractions of citric acid and ferric chloride,however,were only efficient for Cd and the soil pH was decreased significantly.This study suggests that EDTA and DTPA can be considered as suitable agents to clean up the paddy soils contaminated with potentially toxic metals.

Extraction of Lead, Cadmium and Nickel from Contaminated Soil Using Acetic Acid

The accumulation of heavy metals in soil is a serious environmental problem. It is well known that heavy metals have an affinity for different compartments of soil. The risk associated with the presence of metals in soil is the ability of their transfer in water or plants. In the present research, batch extraction experiments were conducted using acetic acid (AA) as an extractant solution at various concentrations and contact times to determine the best conditions of soil washing process to achieve high heavy metal removal efficiencies. AA was investigated for its applicability for the removal of lead, cadmium and nickel from soil. Batch soil washing experiments were performed on 1.0 g portions of the spiked soil using different concentrations (0.001, 0.005, 0.01, 0.05, and 0.1 mol/L) of AA (CH3COOH) with solid: liquid ratio of 1:10. The results showed that AA extracted greater Pb than Cd and Ni. The extraction was carried out with shaking times from 15 to 180 min. The removal percentage of Pb varies from 42.2%-100% and Cd from 5.2%-31.1% with increasing concentration of AA, while the removal efficiency of Ni was not exceeded about 1%. Comparing with Pb and Cd, the removal efficiency of Ni was very low;this means that the solubility of Ni in AA was very low. It was found that 0.1 mol/L AA for soil washing was effective in removing absorbed Pb from contaminated soil (100% efficiency) at time 15 min. While the efficiency reaching 100% with washing solution of 0.05 and 0.01 mol/L at times 120 and 180 min, respectively. The efficiencies of Cd and Ni extraction were improved when 1 mol/L of AA solution was used (41.3% to 70.6% for Cd and 16.3% to 23.3% for Ni).

Effective Sodium Metabisulfite (Na₂S₂O₅), HCl, Sulfur and Distilled Water for the Removal of Pb, Zn and Cr Contaminated Soil in the Columns Method

This paper presents an evaluation of different dose of Sodium Metabisulfite (0.01 M Na2S2O5), (Na2S2O5) + (0.1 HCl), and Distilled water for the removal of soil contaminated with Pb, Zn and Cr by the column mode. The field soil contained concentrations of Pb (307.31 mg⋅kg−1), Zn (207.77 mg⋅kg−1) and Cr (447.50 mg⋅kg−1). Both (0.01 M Na2S2O5), (Na2S2O5) + (0.1 HCl), and Distilled water were found to be effective on removing (Na2S2O5) Pb, Cr and Zn respectively. (Na2S2O5) + (0.1 HCl) Cr, Zn and Pb respectively. Sulfur Pb, Cr and Zn respectively. The removal rate of Pb, Zn, and Cr varied from 10.35% - 26%, 3.4% - 21.60% and 4.97% - 23.88% for (0.01 M Na2S2O5 respectively. The removal rate of Pb, Zn, and Cr varied from 16.13% - 20.07%, 8.20% - 23.48%, 5.42% - 28.93% for (0.01 M Na2S2O5 + 0.1 M HCl) respectively. The removal rate of Pb, Zn, and Cr varied from 10.20% - 25.5%, 9.55% - 25.13% and 6.04% - 25.54% for (S) respectively.

Remediation of oily soil using acidic sophorolipids micro-emulsion

A sophorolipids(SLs)micro-emulsion in Winsor type Ⅰ form was used for crude oil contaminated soil washing treatment.The micro-emulsion shows higher oil removal rate than SLs aqueous solution and diesel oil.The type Ⅰ micro-emulsion with w(SLs)=6%,w(NaCl)=1%,w(diesel)=13.36% gave a high oil removal rate of 95.6% and the eluate remained in type Ⅰ state.The recovered oil showed lower viscosity,mainly caused by the entering of diesel from the micro-emulsion phase into the oil phase and the lower removal rate of the heavier components,such as the resin and asphaltene.The initial heavily saline-alkaline soil changed into mildly saline-alkaline state after washing treatment,favoring the germination and growth of plants,with ryegrass showing better germination and growth effect than alfalfa.The ryegrass showed good phytoremediation effect on the contaminated soil after SLs micro-emulsion washing.The combination process of SL micro-emulsion washing and ryegrass phytoremediation is prospective for oily soil treatment.

螯合剂与低分子有机酸复配淋洗修复Cr(Ⅵ)污染土壤

为探究可生物降解螯合剂氨三乙酸(NTA)替代乙二胺四乙酸二钠(EDTA)在复配淋洗剂中的作用,本文研究了NTA和EDTA与草酸和酒石酸所配制的复配淋洗剂在不同浓度、固液比、pH值和淋洗时间下对Cr(Ⅵ)污染土壤淋洗效果的影响及淋洗动力学,此外还使用顺序提取方案(BCR)分析了淋洗前后土壤中重金属的形态、迁移率和稳定性变化。结果表明:混合等体积的0.25 mol·L^(-1)NTA和0.3 mol·L^(-1)草酸在固液比为1∶9、pH为3、淋洗时长为240 min条件下达到最大Cr(Ⅵ)去除效率82.94%,这与使用EDTA淋洗时的效率相近;该复配淋洗剂淋洗Cr(Ⅵ)污染土壤时的动力学符合拟二级动力学方程;BCR实验结果表明,NTA和草酸复配淋洗后土壤中Cr的弱酸可提取态、可氧化态和残渣态组分含量分别从6346.67、4280.00、2387.67 mg·kg^(-1)下降至1161.67、433.35、741.00 mg·kg^(-1),可还原态组分从2746.67 mg·kg^(-1)上升至6016.67 mg·kg^(-1),污染土壤重金属迁移率下降。淋洗后土壤有机质含量从13.47%上升至30.56%,脲酶和脱氢酶活性分别从46.32、329.09μg·d^(-1)·g^(-1)降低至29.36、131.98μg·d^(-1)·g^(-1),土壤结构基本与原土一致。因此,NTA与草酸复配淋洗剂是一种高效且对土壤影响较小的淋洗剂。

铁盐淋洗剂对稀土污染土壤的淋洗修复研究

稀土矿的大量开采,加剧了土壤污染,危害居民健康。本研究采用不同三价铁盐(Fe^(2)(SO_(4))_(3)、Fe(NO_(3))_(3)和FeCl_(3))对稀土污染的农田土壤进行淋洗修复,同时对比其他无机盐(亚铁盐、镁盐、铵盐和钠盐)的淋洗效果;并研究三价铁盐淋洗剂浓度、液固比和淋洗时间对稀土元素(REE)去除效果的影响。结果表明,三价铁盐对REE的淋洗率高达41.7%~54.3%,显著高于其他无机盐(0.2%~26.8%);三价铁盐的最佳淋洗时间为1440 min,当淋洗剂浓度从5 mmol/L增加到50 mmol/L的过程中,土壤中REE淋洗率显著增加,淋洗率随淋洗液固比(1∶1~5∶1)的升高而增加,其中,淋洗剂浓度对REE去除率的促进作用显著大于液固比。在优化淋洗条件下,Fe_(2)(SO_(4))_(3)对REE的淋洗率为54.3%,显著高于Fe(NO_(3))_(3)(46.1%)和FeCl_(3)(41.7%),这主要是由于REE^(3+)与SO_(4)^(2−)易发生络合,促进了REE的溶出。三价铁盐对土壤中REE的高效去除,主要是由于酸解作用、较强的阳离子交换能力,以及阴离子配体与稀土元素的络合作用。此外,三价铁盐对MREE的去除效率显著高于LREE土和HREE,表现出明显的分异特征。上述研究可为稀土污染土壤的修复治理提供指导与借鉴。

Evaluation of soil washing process with carboxymethyl-β-cyclodextrin and carboxymethyl chitosan for recovery of PAHs/heavy metals/fluorine from metallurgic plant site

Polycyclic aromatic hydrocarbons（PAHs）/heavy metals/fluorine（F） mixed-contaminated sites caused by abandoned metallurgic plants are receiving wide attention. To address the associated environmental problems,this study was initiated to investigate the feasibility of using carboxymethyl-β-cyclodextrin（CMCD） and carboxymethyl chitosan（CMC） solution to enhance ex situ soil washing for extracting mixed contaminants. Further,Tenax extraction method was combined with a first-three-compartment model to evaluate the environmental risk of residual PAHs in washed soil. In addition,the redistribution of heavy metals/F after decontamination was also estimated using a sequential extraction procedure. Three successive washing cycles using50 g/L CMCD and 5 g/L CMC solution were effective to remove 94.3% of total PAHs,93.2% of Pb,85.8% of Cd,93.4% of Cr,83.2% of Ni and 97.3% of F simultaneously. After the 3rd washing,the residual PAHs mainly existed as very slowly desorbing fractions,which were in the form of well-aged,well-sequestered compounds; while the remaining Pb,Cd,Cr,Ni and F mainly existed as Fe–Mn oxide and residual fractions,which were always present in stable mineral forms or bound to non-labile soil fractions. Therefore,this combined cleanup strategy proved to be effective and environmentally friendly.

石油烃污染土壤修复的有效方法:表面活性剂洗涤

随着对能源需求的增加,石油资源被广泛开发和利用,不可避免地造成了环境污染。石油是一种常见的土壤污染物,对环境造成严重危害,因此,对石油污染土壤进行补救是一项艰巨的任务。表面活性剂清洗是一种有效的补救手段,表面活性剂可以促进土壤中石油烃的解吸。本文总结了利用基于表面的化学洗涤方法对石油污染土壤进行修复的原理和方法,综述了用于清洗油污土壤的各种类型的表面活性剂及其混合成分,并简要讨论了影响洗涤过程的环境因素。

Comparison of a plant based natural surfactant with SDS for washing of As(V) from Fe rich soil

This study explores the possible application of a biodegradable plant based surfactant, obtained from Sapindus mukorossi, for washing low levels of arsenic （As） from an iron （Fe） rich soil. Natural association of As（Ⅴ） with Fe（Ⅲ） makes the process difficult. Soapnut solution was compared to anionic surfactant sodium dodecyl sulfate （SDS） in down-flow and a newly introduced suction mode for soil column washing. It was observed that soapnut attained up to 86% efficiency with respect to SDS in removing As. Full factorial design of experiment revealed a very good fit of data. The suction mode generated up to 83 kPa pressure inside column whilst down-flow mode generated a much higher pressure of 214 kPa, thus making the suction mode more efficient. Micellar solubilisation was found to be responsible for As desorption from the soil and it followed 1st order kinetics. Desorption rate coefficient of suction mode was found to be in the range of 0.005 to 0.01, much higher than down-flow mode values. Analysis of the PT-IR data suggested that the soapnut solution did not interact chemically with As, offering an option for reusing the surfactant. Soapnut can be considered as a soil washing agent for removing As even from soil with high Fe content.

Removal of heavy metals and arsenic from a co-contaminated soil by sieving combined with washing process

Batch experiments were conducted with a heavy metals and arsenic co-contaminated soil from an abandoned mine to evaluate the feasibility of a remediation technology that combines sieving with soil washing.Leaching of the arsenic and heavy metals from the different particle size fractions was found to decrease in the order：〈 0.1,2–0.1,and 〉 2 mm.With increased contact time,the concentration of heavy metals in the leachate was significantly decreased for small particles,probably because of adsorption by the clay soil component.For the different particle sizes,the removal efficiencies for Pb and Cd were75%–87%,and 61%–77% for Zn and Cu,although the extent of removal was decreased for As and Cr at 〈 45%.The highest efficiency by washing for Pb,Cd,Zn,and As was from the soil particles 〉 2 mm,although good metal removal efficiencies were also achieved in the small particle size fractions.Through SEM-EDS observations and correlation analysis,the leaching regularity of the heavy metals and arsenic was found to be closely related to Fe,Mn,and Ca contents of the soil fractions.The remediation of heavy metal-contaminated soil by sieving combined with soil washing was proven to be efficient,and practical remediation parameters were also recommended.

废弃物发酵液淋洗去除土壤重金属的效率、机理及生态风险

为寻找修复重金属污染土壤的高效、绿色材料,本实验采用菠萝皮渣、哈密瓜皮渣等农业废弃物,并添加乳酸菌进行发酵制得富含有机酸的高效淋洗剂,并分析淋洗剂浓度、pH、淋洗时间以及淋洗次数等参数对土壤重金属镉(Cd)、铅(Pb)、锌(Zn)的去除效率、机理及生态风险。结果表明:与未发酵浸提液相比,乳酸菌发酵液可以提升土壤Cd、Pb、Zn的去除率14.27%~20.76%;且随着淋洗剂浓度增加、pH降低、淋洗时间延长及淋洗次数的增加,Cd、Pb、Zn的去除率显著提升;菠萝皮渣发酵液淋洗3次后,土壤中残留的Cd和Zn含量均低于国家土壤质量标准(GB 15618—2018)中农用地的限值,表明利用乳酸菌发酵制得的淋洗剂可作为一种绿色高效的土壤重金属修复材料。发酵液多次淋洗后,土壤中Cd、Zn的环境风险和生物有效性降低,移动性减弱,稳定性增强;而重金属Pb总量降低,但移动性增强,其安全性仍值得关注。

Effect of co-presence of NSAIDs with cadmium:i)evaluation of NSAIDbearing water for washing Cd from soil,ii)Cd removal from NSAIDbearing water using magnetic graphene oxide

Cadmium“Cd”is a toxic pollutant that may present in soil and water.This work evaluates:i)the use of non-steroidal anti-inflammatory drugs“NSAIDs”-bearing water for washing soil containing Cd(Ⅱ),ii)removal of Cd(Ⅱ)from NSAID-bearing water by adsorption onto magnetic graphene oxide which can be easily separated by strong magnet.The studied NSAIDs are aspirin,ketoprofen,ibuprofern and diclofenac.The Cd(Ⅱ)-NSAIDs complexes were synthesized and characterized by FT-IR.Graphene was initially oxidized by either nitric acid,or ammonium persulphate method,or Hummer's method.Magnetite was then deposited on graphene oxide to give the corresponding magnetic graphene oxides(NA-MGO,APSMGO and Hum-MGO,respectively).The effect of the following factors on Cd(Ⅱ)uptake was investigated:NSAIDs type,pH,graphene oxidation method,magnetite:graphene oxide mass ratio in the adsorbent,(Cd(Ⅱ):NSAID)molar ratio.Maximum Cd(Ⅱ)uptake was achieved using“magnetic graphene oxidized with ammonium persulphate where the mass ratio of magnetite to graphene oxide was 2:1”in the presence of diclofenac at pH6.The best Cd(Ⅱ):diclofenac molar ratio was 1:3.The maximum adsorption capacity of Cd(Ⅱ)was found to be 83 mg L1.The regeneration of the adsorbent was possible by 0.3 M HNO3 solution and 80%of adsorption efficiency was maintained after five cycles.Upon presence of coexisting ions,80%of the adsorption efficiency was maintained.Various NSAIDs-containing waters were used for washing Cd-containing soil;the maximum removal efficiencies of Cd were 18%and 16%using 5 mM diclofenac or 10 mM aspirin,respectively.Using diclofenac or aspirin-spiked real pharmaceutical wastewater gave 28%removal of Cd.The optimum adsorption method was used for removal of Cd(Ⅱ)from diclofenac-containing soil-washing water,where two successive adsorptions were needed for complete Cd uptake.

Efficient Removal of Lead from Washing Effluent of Lead-contaminated Soil with Garlic Peel

Wastewater produced from the soil washing process contains heavy metals, which limits its reuse for washing. So it is necessary to develop an efficient and economical way to recycle it, and this study presented a biosorption method to realize this goal. A typical soil sample contaminated by lead was taken from the real field near a lead smelting factory, used for the toxic metals extraction with dilute citric acid. A leach liquor was obtained with lead ions at the level of 12.35 mg/L, Cd 1.2 mg/L, Cu 1.5 mg/L, Zn 2.6 mg/L, as well as the coexisting anions, such as sulphate, silicate, chloride at the concentration of several hundred miligram per liter. The garlic peel was modified by a simple chemical saponification process and used as the biosorbent for toxic metal removal. Firstly, the adsorp- tion behavior of lead ions on the saponified garlic peel was systematically investigated using the synthetic solutions, and then the adsorption mechanisms were explored by detailed experhnents combining with the thermodynamic calculation reuslts of the aqueous system of Pb（II）-citrate-H2O. It was found that in artificial solution containing 0.01 mol/L citrate, the maximum adsorption capacity of 261.0 mg/g was reached at pH near 3.0, and also at this very pH value the Pb^2＋ and Pb（H2Cit）＋ were the dominant lead species, which are favorable for adsorption due to its easier approaching to the --COO^- ligands in the saponified garlic peel partilces via charge attraction, and the appearance of Pb（HCit）; and Pb（Cit）- at pH above 3.0 inhibits the adsorption. Secondly, the real leach liquor was used for adsorption tests, and twice adsorption under the optimal conditons would decrease the residual concentrations of Pb, Cd, Cu and Zn to zero. After elution by using 0.1 mol/L nitric acid, the adsorbed metals can be recovered and garlic peel can be reused for at least 10 cycles effectively. This study presents a prospective biosorption method for economical and efficient removal of the lead ions from soil washing wastewater with citric acid as the leaching reagent.

Washing out heavy metals from contaminated soils from an iron and steel smelting site

Washing is a promising method for separating contaminants bound to the particles of soil ex-situ by chemical mobilization. Laboratory batch washing experi- ments were conducted using deionized water and varying concentrations of oxalic acid, citric acid, tartaric acid, acetic acid, hydrochloric acid and ethylenediaminetetra acetic acid （EDTA） to assess the efficiency of using these chemicals as washing agents and to clean up heavy metals from two heavily polluted soils from an iron and streel smelting site. The toxicity reduction index and remediation costs were analyzed, and the results showed that the soils were polluted with Cd, Pb and Zn. Hydrochloric acid and EDTA were more efficient than the other washing agents in the remediation of the test soils. The maximum total toxicity reduction index showed that 0.5 mol·L^-1 hydro- chloric acid could achieve the remediation with the lowest costs.