Use of batch leaching tests to quantify arsenic release from excavated urban soils with relatively low levels of arsenic

Purpose

In Japan, the excavated soils produced from constructions projects, which contain relatively low levels of arsenic (As), are considered as a potential concern if they could release significant amount of As to the environment. The aim of this study was to investigate the As leaching from excavated alkaline soils and, in particular, the influence of drying methods, pH of extracting solution, and consecutive washing on As leaching from these soils.

Materials and methods

Four excavated alkaline soils obtained from different construction sites in Tokyo, Japan, were used in this study. The soils were pretreated by three drying methods (air-dried, 40 °C-dried, and freeze-dried). Sequential extraction procedure was applied to partition As into five operationally defined chemical fractions. Batch leaching tests (initial pH-controlled leaching test and consecutive washing test) were conducted to investigate the As release under different leaching conditions.

Results and discussion

The As contents in the four soils were 9.22, 79.4, 6.75, and 11.7 mg kg^?1, respectively, and As was primarily associated with the residual phase. Arsenic leaching was strongly dependent on the extracting solution pH values. Strongly acidic extracting solution (pH 2) led to circumneutral leachates and limited As mobility, whereas the strong alkaline-extracting solution (pH 12) greatly enhanced the As release from these soils. The consecutive washing test results revealed a long-term release of As from these excavated soils. The pollution potential indices (PPIs) were successfully used to evaluate the pollution threat of As leaching from excavated soils. In addition, different drying methods resulted in variations in the short- and long-term release of As from these excavated soils.

Conclusions

The results revealed that the soil pretreatment and the leaching conditions should be considered if we want to use batch tests for the contamination assessment of excavated urban soils from construction projects. Different drying methods and single extraction may lead to misestimation of the As pollution level. High extraction efficiency with strong alkaline-extracting solution (pH 12) reveals that it could potentially be used to wash As from excavated alkaline soils.

     

A Comparison of Water Treatment Sludge and Red Mud as Adsorbents of As and Se in Aqueous Solution and Their Capacity for Desorption and Regeneration

The adsorption of As(III), As(V), Se(IV) and Se(VI) by seawater neutralized red mud and alum water treatment sludge was investigated and compared using the batch adsorption technique. For water treatment sludge, adsorption of As(V), Se(IV) and Se(VI), at equimolar concentrations of added metalloid, declined with increasing pH. The decline was rapid above pH?4.0 for Se(VI), above pH?5.0 for Se(IV) and above pH?6.0 for As(V). Adsorption of As(III) increased with increasing pH up to pH?9.0 and then declined. For red mud, adsorption of As(V), Se(IV) and Se(VI) showed a maximum at about pH?5.0 and for As(III) adsorption remained relatively constant over the pH range 2.0?C10.0 after which it declined. Water treatment sludge removed 50?% or more of solution As(V) between pH?2.0 and 10.8, Se(IV) between 2.0 and 8.9, Se(VI) between 2.0 and 5.8 and As(III) between 8.4 and 10.9. By contrast, red mud showed less than 25?% adsorption of added Se(VI) and As(III) over the entire pH range tested (2.0?C12.0) and reached 50?% or more for As(V) only over the pH range 4.0?C6.9 and for Se(IV) between pH?4.3 and 5.6. At pH?5.0, adsorption of As(III) and Se(IV) was better described by the Langmuir than Freundlich equation but the reverse was the case for As(V) and Se(VI). Kinetic data for adsorption of all four oxyanions onto both adsorbents correlated well with a pseudo-second-order kinetic model suggesting the process involved was chemisorption. NaOH was more effective at removing adsorbed metals from both adsorbents than HNO₃. Water treatment sludge maintained its As(III) and Se(IV) adsorption capability at greater than 70?% of that added over eight successive cycles of adsorption/regeneration using 0.5?M NaOH as a regenerating agent. By contrast, for red mud, As(V) adsorption capacity declined very rapidly after three adsorption/desorption cycles and that for Se(IV) it decreased progressively with increasing numbers of cycles. It was concluded that water treatment sludge is a suitable material to develop as a low-cost adsorbent for removal of heavy metal oxyanions from wastewater streams.     

Contaminant Mobility in Soils Amended with Fly Ash and Flue-Gas Gypsum: Intact Soil Cores and Repacked Columns

The impact of the land application of coal combustion by-products, fly ash (FA) and flue-gas desulfurization gypsum(FDG), to coarse-textured soils of the southeastern U.S.A. wasinvestigated using batch and dynamic column techniques. Two FAsamples, one an alkaline FA (Alk-FA) and the other an acidic FA(Acid-FA), were evaluated alone and in combination with FDG assoil amendments to an Appling loamy sand (Typic Hapludults). Theeffects of these waste products on clay dispersion, soilhydraulic conductivity (K_sat) and the migration ofcontaminants such as Arsenic (As) and Boron (B) were studiedusing intact soil cores and repacked soil columns. FA or combinationsof FA + FDG were applied to the surface of intact soil cores (10 Mg ha^-1) and repacked soil columns or incorporated withinrepacked soil columns. The columns were saturated and thenleached for a prescribed number of pore volumes to simulateleaching conditions in the field. Effluent pH, electricalconductivity (EC), and turbidity were monitored and leachatefractions were collected for B, As, Ca, Mg, K and Na analysis.Both FA materials were ineffective at decreasing the inherentdispersibility of clay from the A_p horizon in batch tests.In fact, high application rates of the Alk-FA induced some claydispersion in the well-flocculated B_t soil materials, andcolumn results suggest that incorporating the Alk-FA within thesurface soil may actually reduce K_sat. In contrast,treatments with FDG were highly effective at inducing rapid clayflocculation in batch tests and eliminating effluent turbidityfor intact and repacked soil columns. Boron was readily mobilefrom both intact and repacked soil columns, a majority of whichleached from the columns within the first three pore volumes.Boron leaching was greater for combined treatments (FA + FDG),possibly indicative of enhanced solubilization in the presenceof FDG or sulfate (SO₄ ^2-) competition for sorptionsites. Arsenic levels present in the leachates from FA and FDGcolumns were generally lower than control columns and roughlycorrelated with effluent turbidity. Combined treatments (FA +FDG) enhanced Mg and K leaching due to the added competition ofCa for cation exchange sites. Following leaching, the intactsoil cores were sectioned at 5 cm intervals and the pH and EC ofthe soil, as well as the vertical distribution of As and B, weredetermined. Levels of residual As were only slightly higher in the upper section of the FA-amended columns, showing little downward movement, but no clear trend in residual B was observed due to its greater mobility.     

GLDA淋洗参数对污泥中重金属去除效率的影响及其动力学分析

采用GLDA作为淋洗剂对工业污水污泥进行淋洗,确定淋洗的最优参数,为治理工业污泥提供参考依据。通过振荡淋洗研究了GLDA浓度（0.5% ~ 20%）、pH（2 ~ 10）、液固比（10∶1 ~ 80∶1）和淋洗时间（0 ~ 24 h）对污泥中重金属去除率的影响,利用动力学方程对实验数据进行拟合。结果表明,降低pH、增加液固比和延长淋洗时间均可提升重金属的淋洗效率,在pH为2,浓度5%,液固比60∶1,淋洗时间480 min时,GLDA对污泥中的重金属淋洗率达到最优,镍（Zn）、镍（Ni）、镉（Cd）和铬（Cr）的去除率分别为91.21%、76.34%、69.56%和65.01%。准二级动力学方程可以较好的拟合GLDA对4种重金属的淋洗过程,说明重金属淋洗过程以化学淋洗为主。综上所述,在进行淋洗参数优化的基础上,GLDA可用于工业污水污泥中重金属的去除。     

Modeling and Evaluating the Performance of River Sediment on Immobilizing Arsenic from Hydrothermally Altered Rock in Laboratory Column Experiments with Hydrus-1D

Large volumes of excavated rock are produced as a result of road and railway tunnel construction in Hokkaido, Japan. Due to the geological condition of this region, these rocks have often undergone hydrothermal alterations, causing them to contain elevated amounts of hazardous elements including arsenic (As). Therefore, these excavated rocks are potentially hazardous waste, and proper disposal methods are required. In this article, performance of unsaturated river sediment on immobilizing As from hydrothermally altered rock is evaluated using laboratory column experiments and Hydrus-1D. The results reveal that the river sediment significantly reduces As migration. Arsenic retarded by river sediment was observed in three patterns. The first was an adsorption onto minerals originally contained in the river sediment. The next pattern was a combination of reduction of As generation by oxidation of As bearing-minerals, irreversible adsorption, and adsorption onto newly precipitated Fe oxy-hydroxide/oxide. The last pattern led to a further depletion of As leached from the rock layer due to a shift in the majority of the As generation mechanism from dissolution to oxidation in combination with a low concentration of oxygen in the rock layer. These patterns were satisfactorily evaluated by a Hydrus-1D model with reversible and irreversible adsorptions. The information from this work is effective in designing and establishing a reasonable technique for the disposal of hydrothermally altered rocks.     

Feasibility of a counter-current extraction procedure for the removal of heavy metals from contaminated soils

Effective remediation and sanitation technologies for soils contaminated with heavy metals are limited. We investigated the feasibility of a counter-current metal extraction procedure for the removal of selected heavy metals (Cd, Cu, Ph, and Zn) from two contaminated soils. The process involved a decarbonation (removal of carbonates), acid solubilisation, washing, and liming step. Results from batch equilibration experiments simulating the counter-current process showed more than 85% of the Cd present to be removed. Removal efficiencies for Cu and Pb were limited to approximately 15%, this mainly due to resorption of these elements during the decarbonation step. As most Zn was found to be present in a more difficult acid-extractable solid phase, its extractability accounted for only 25%. While reaction (pH) conditions of both decarbonation and solubilisation determined removal efficiencies, washing the extracted soil with deionized water only slightly increased the amount of metals removed. Metal distribution among solid phases — exchangeable, carbonate, reducible, organically bound, and residual — was affected by the different treatments. The amount of metals contained in the exchangeable and residual fractions determined their extractability. Except for Cu, the reducible and organically bound fractions were less important. After solubilisation 13 to 70% of the metals were present in an exchangeable solid phase. This implicates that washing the solubilized soil with a salt may increase the extractability of metals, especially for Zn and Pb. Based on our results the process is critically evaluated and possibilities for optimization formulated.     

砂质土壤磷素的组成特点与释放规律研究

砂质土壤磷素主要以可提取态形式积累,有很高的释放潜力。该类土壤磷素的释放受土壤pH、土水作用时间和土壤溶液化学组成等的影响。土壤酸化、土壤溶液中Na+浓度的提高及土壤与水的作用时间增加可促进土壤磷素的释放。用淋洗方法和平衡提取等2种方法对土壤磷素的释放评价表明,淋洗方法测得的P量较小,可代表土壤短期内P释放强度;而平衡提取法提取的P数量较大,可代表土壤P长期释放的容量。     

EDTA Leaching of Cu Contaminated Soils Using Ozone/UV for Treatment and Reuse of Washing Solution in a Closed Loop

Ozone and UV irradiation were used for oxidative decomposition of EDTA-Cu complexes in washing solution obtained during multi-step leaching of Cu (344,1?±?36.5 mg kg^?1) contaminated vineyard soil with EDTA as a chelant. The released Cu was absorbed from the washing solution on a commercial mixture of metal absorbing minerals, and the treated washing solution then reused for removal of soil residual Cu-EDTA complexes in a closed-loop process. Six consecutive leaching steps (6?×?2.5 mmol kg^?1 of EDTA) removed 38.8 % of Cu from soils, and reduced Cu soil mobility, determined using the toxicity characteristic leaching test (TCLP), by 28.5%. The final washing solution obtained after soil remediation was colourless, with a pH close to neutral (7.5?±?0.2) and with low concentrations of Cu and EDTA (0.51?±?0.22 mg L^?1 and 0.083 mM, respectively). The proposed remediation method has therefore potential not just to recycle and save process water, but also not to produce toxic wastewaters. Soil treatment did not substantially alter the soil properties determined by pedological analysis, and had relatively little impact on soil hydraulic conductivity and soil water sorption capacity.     

Factors affecting reduction of selenate to elemental selenium in agricultural drainage water by Enterobacter taylorae

Microbial reduction of selenate [Se(VI)] to elemental selenium [Se(0)] is a useful technique for removing Se from agricultural drainage water. A series of batch experiments were conducted in the laboratory to determine the effects of yeast extract (50-1000 mg/L), salinity (EC, 5-75 dS/m), and NO(3)(-) (5-100 mg/L) on the removal of Se(VI) (2000 microg/L) from drainage water by Enterobacter taylorae. Results showed that relatively high amounts of yeast extract (500 mg/L) were needed for E. taylorae to effectively reduce Se(VI) to Se(0). During a 7-day experiment, approximately 95% of added Se(VI) was reduced to Se(0) in the low-salinity drainage water (5 dS/m) with NO(3)(-) values of 5-50 mg/L. In the high-salinity drainage water (50-75 dS/m), reduction of Se(VI) to Se(0) was limited. E. taylorae was also capable of reducing Se(VI) to Se(0) in the San Joaquin Valley drainage water, with a reduction of the added Se(VI) to Se(0) (73.8%) and Se(-II) (20%). This study suggests that E. taylorae may be used to treat Se(VI)-contaminated drainage water in the field.     

Arsenic Removal from Drinking Water using Iron Oxide-Coated Sand

This article describes experiments in which iron oxide-coated sand(IOCS) was used to study the removal of both As(V) and As(III) to a level less than 5 μg L^-1 in drinking water. Iron oxide-coated sand 2 (IOCS-2) prepared through high temperature coating process was used in batch and column studies to assess the effectiveness and suitability. The isotherm study results showed that the observed data fitted well with the Langmuir model, and the adsorption maximum for IOCS-2 at pH 7.6 was estimated to be 42.6 and 41.1 μg As g^-1 IOCS-2 for As(V) and As(III), respectively. In the fixed bed column tests to study arsenic removal from the tap water, good performance ofIOCS-2 was observed in respect of bed volumes achieved and arsenic removal capacity. Five cycles of column tests were conducted to evaluate the performance of IOCS-2, and arsenic wassuccessfully recovered from the media through regeneration and backwash operations. High bed volumes (860 to 1403) up to a breakthrough concentration of 5 μg L^-1 were achieved inthe column studies with tap water, and the bed volumes achievedin the studies with natural water (containing arsenic) were 1520.The results of both the batch and column studies showed that ironoxide-coated sand filtration could be effectively used to achieveless than 5 μg L^-1 As in drinking water.     

Assessment of seafood processing wastes as alternative sources of Selenium in plant production

Abstract

In some parts of the world, the soil selenium (Se) content is too low to ensure the Se level recommended for human or animal consumption in the crops produced. In order to secure a desired concentration of Se in crops, Se has been applied as mineral fertilizer to agricultural fields. Since only a minor part of the inorganic Se applied is utilized by plants and small increases in Se concentrations in, e.g., drinking water, may be toxic, the method is somewhat controversial. As an alternative to Se-enriched mineral fertilizer, different seafood-processing wastes have been examined as a source for Se in crop production. Both in greenhouse pot experiments and field trials the Se in seafood waste was not plant-available during the first growing season. There was no significant difference between the Se concentration in wheat growing in soil without added Se and in soil receiving Se from seafood waste in amounts ranging from 0.9 to 9 g ha^?1. Neither was any residual effect of Se in seafood waste seen during a second year growth period. Thus, seafood-processing waste cannot be regarded as a potential source of Se in crop production. Possible mobilization of formerly applied Se, as seafood-processing waste or Se enriched mineral fertilizer due to changes in soil redox conditions were examined in a leaching experiment. The mobility of formerly applied Se was generally very low, but the results indicated that under permanently wet soil conditions leaching of Se may occur in plant dormant periods in soils with low organic matter content and high pH.     

Effects of Additional Layer(s) on the Mobility of Arsenic from Hydrothermally Altered Rock in Laboratory Column Experiments

Hydrothermally altered rocks are frequently encountered when tunnels are constructed in Hokkaido, Japan. High concentraions of hazardous elements, such as arsenic (As), are often released from these rocks into the surrounding environments. Therefore, the rocks are considered potentially hazardous waste. This article describes the effects of water content and oxygen (O₂) concentration in relation to additional layer(s), i.e., surface covering and bottom adsorption layers, on As leaching by using laboratory columns with water content and O₂ concentration sensors. The results show that the use of additional layer(s) has a significant effect on lowering As migration. This was due not only to the adsorption capacity of As by the adsorption layer but also to the water content and O₂ concentration inside the rock layer. The accumulation of pore water was increased in the rock layer in cases with additional layer(s), which resulted in lower O₂ concentration in the rock layer. Consequently, the leaching of As by the oxidation of As-bearing minerals in the rock layer was reduced. Moreover, a longer water-resident time in the rock layer may induce precipitation of Fe oxy-hydroxide/oxide. These results suggest that the geochemical conditions of the rock layer affect As leaching and migration.     

煤矸石土壤砷污染相关性分析

在对内蒙古自治区某矿区周边土壤环境总砷含量、全磷、有机质、含水率等项目测定的基础上,分析了土壤总砷含量、有效砷含量与土壤理化性质指标的相关性。同时,通过模拟研究区降水特点和降水量,采用淋溶试验装置,对由煤矸石和自然表层土壤组成的两组混合样品在不同pH值条件下砷的析出规律进行了实验研究。结果表明,研究区土壤有效态砷与总砷含量存在显著正相关关系;总砷含量与pH值、全磷存在一定的正相关性,与有机质和水解性氮之间的相关性不显著;土壤有效态砷含量与有机质含量呈正相关关系,与pH值呈一定的负相关关系。混合土层中砷的淋出量随淋溶液pH值的增加呈降低趋势,pH值越低,砷淋出量越高。     

Comparability of and Alternatives to Leaching Tests for the Assessment of the Emission of Inorganic Soil Contamination (11 pp)

Goal, Scope and Background

The suitability of leaching tests for the assessment of soil water concentrations is a controversial issue. The aim of this paper is to review and critically discuss the applicability and comparability of leaching tests in the scope of groundwater risk assessment of inorganic contamination and to discuss soil water sampling methods as alternative.

Main Features

First, the theoretical background of leaching tests and the main leaching controlling parameters (pH, redox, ionic strength, reaction kinetics, surface, and speciation) are discussed. Experience with common batch leaching tests (the German DEV S4 test (S4), saturation soil extraction (SSE), ammonia nitrate extraction (ANE), and pHstatic tests) are compiled and an emphasis is set on the comparability of the results of batch leaching tests. Additionally, the comparability between batch tests and column tests is discussed and comparison studies are reviewed. As a feasible alternative, soil water sampling strategies (the suction cup method and centrifugation) are outlined.

Results and Conclusions

For an expedient application of leaching tests, it is crucial to identify the main release controlling parameters, which can be both site and contaminant specific. Possible controlling parameters are, for example, pH, redox conditions, specific surface area of the investigated material, liquid to solid-ratio, ionic strength, cation exchange capacity, DOC, speciation, temperature and contact time. It depends on the release process of the contaminant in the investigated material, which parameters are influencing the release predominantly. The following questions have to be answered: Is the release process availability controlled or solubility controlled and are there kinetic limitations? Suction cups are particularly useful for long-term monitoring. In contrast, centrifugation is a fast and simple method to sample soil water also at larger and heterogeneous sites. SSE or similar batch tests for coarse material are suitable if the soil water cannot be investigated directly. Contaminant concentrations of the SSE fit best to contaminant concentrations in soil water compared to other leaching tests. Concentrations measured with S4 tests are usually significantly lower than in real soil water due to the unrealistically high liquid to solid-ratio. The ANE is used for the evaluation of plant uptake. It does not represent realistic soil water concentrations. Cationic contaminants are usually significantly higher concentrated in ANE eluates. pHstatic tests provide an evaluation of the pH dependency of contaminant release and buffer capacity of the investigated material. It provides indications to release processes and often explains deviations in the results of different leaching tests. Previous practical experience and fundamental research studies show that a conversion of leaching test results, as is proposed by the (already withdrawn) DIN V 19735, is impossible. Correlations of results from different methods, regarding different kinds of materials and different sites, are not significant. This is due to different leaching processes.

Recommendation

. For short-term risk assessment, soil water should be sampled and investigated directly by suction cups (for monitoring purposes) or centrifugation (for inventory purposes). If this is not possible, the SSE or analogous batch tests for coarse material with a narrow L/S ratio should be applied. A suggestion could be a modified saturation soil extraction where a soil column is saturated and the eluate is removed by suction at the bottom of the column. With this method, an unsaturated column test could follow in the same vessel. In order to assess the long-term leaching behavior, total contents of the investigated material have to be taken into account additionally. Furthermore, it is essential to understand the dominant physical and chemical release processes and to figure out the main leaching controlling parameters. Therefore, the following methods are recommended: pHstatic tests at different pH values provide an insight to leaching processes and possible future leaching scenarios. Batch tests similar to the S4 procedure with different L/S ratios are useful to find out whether the release process is solubility or availability controlled. Additionally, this method allows the determination of maximum solubility and maximum availability, respectively. Furthermore, unsaturated column tests provide an insight into leaching processes and releasable amounts even though they require great experimental effort. Other leaching tests like S4 or ANE are not suitable for the estimation of soil water concentrations.     

Mixed chelators of EDTA,GLDA, and citric acid as washing agent effectively remove Cd,Zn, Pb,and Cu from soils

Purpose

Soil washing with chelators is a viable treatment alternative for remediating multi-contaminated soils. The aim of this study was to investigate the removal efficiencies of Cd, Zn, Pb, and Cu in alkaline and acid multi-metal-contaminated soils by washing with the mixed chelators (MC).

Materials and methods

The batch experiments were carried out to evaluate the removal efficiencies of heavy metals in contaminated soils by the MC with different molar ratios of EDTA, GLDA, and citric acid, and evaluated the washing factors, including contact time, pH, MC concentration, and single and multiple washings at the same MC dose, on the removal efficiencies.

Results and discussion

Results showed that the removal efficiencies for Cd, Zn, Pb, and Cu by the MC (the molar ratio of EDTA, GLDA, and citric acid was 1:1:3) were as much as those of the only EDTA washing from both soil at the same application dose of total chelators; moreover, the application dose of EDTA decreased by 80%. For the alkaline-contaminated soil, the removal efficiencies of Cd, Zn, Pb, and Cu decreased with the increasing of the solution pH, which was opposite to acid-contaminated soil. This was attributed to that the metal-ligand complex could be obviously re-adsorbed on the soil surface sites, particularly in low pH values. The removal efficiencies of Cd, Zn, Pb, and Cu depended on MC concentration. A higher MC concentration led to a more effective removal of Cd, Zn, Pb, and Cu in alkaline-contaminated soil; however, their changes were slightly increased in acid-contaminated soil. At the same dose of MC, single washing with higher MC concentration might be favorable to remove heavy metals, moreover, with much less wastewater generation.

Conclusions

The MC (the molar ratio of EDTA, GLDA, and citric acid was 1:1:3) may be a useful, environmentally friendly, and cost-effective chelators to remediate heavily multi-metal-contaminated soil.

     

A Novel Pretreatment Method of Lignocellulosic Material as Adsorbent and Kinetic Study of Dye Waste Adsorption

Sulphuric acid-modified bagasse has been used as low-cost adsorbent for the removal of methylene blue (MB) dye from aqueous solution. In order to remove organic compounds that contribute to chemical oxygen demand (COD), pretreatment with thorough washing of adsorbent using boiling distilled water was performed instead of conventional washing using distilled water at room temperature only. This has resulted in the highest efficiency of color removal of 99.45% and COD reduction of 99.36% for MB dye solution at pH 9. Effects of initial pH, dye concentration, adsorbent dosage, temperature, and contact time have been studied. The adsorption of MB dye was pH dependent. Langmuir and Freundlich isotherm models were tested on the adsorption data. The kinetic experimental data were analyzed using pseudo-first order, pseudo-second order, and the intraparticle diffusion model in order to examine the adsorption mechanisms. The adsorption process followed the Langmuir isotherm as well as the Freundlich isotherm and pseudo-second-order kinetic model. The process was found to be endothermic in nature.     

石灰水对混合试剂洗脱土壤重金属淋洗效果的影响

为了探索石灰水对混合试剂(MC)洗脱后土壤中重金属的淋洗效果,对重金属污染土壤进行了盆栽淋洗试验研究。结果表明:石灰水淋洗MC洗脱土壤,淋出液中Cd、Zn、Pb和Cu浓度均显著高于去离子水淋洗MC洗脱土壤,这主要由于石灰水提高土壤pH,释放土壤所吸附的络合态重金属,这种现象在酸性污染土壤中表现更明显。对于酸性污染土壤,可采用MC+石灰水+去离子水依次进行淋洗处理,其Cd和Zn去除率与MC+去离子水+去离子水依次淋洗处理相比提高22.6%和28.8%;对于中性重金属污染土壤,两处理间重金属去除率差异不显著。石灰水淋洗可提高土壤重金属去除率,并提高土壤pH,改善MC洗脱对土壤的酸化,有利于淋洗后土壤的农业利用。     

皂角苷对几种生活污泥中和的去除

为探讨皂角苷对污泥中Cu和Zn去除的机制,采用化学淋洗的方法研究皂角苷浓度（0~7%）、pH（2~6）、振荡时间（0~48 h）、温度（10、25、40℃）和提取次数（1~4次）对四种生活污泥中Cu和Zn去除率的影响,分析去除前后重金属的形态,并监测去除过程中浸出液pH的变化。结果表明,单次淋洗条件下,皂角苷对Cu的去除率大于Zn;皂角苷溶液的pH对Cu的去除没有显著影响,而Zn的去除率在pH下降到2时明显增加;Cu经过6~12 h的振荡就能达到最大去除,Zn则需要经过较长时间的振荡（48 h）才能达到较好去除;提高振荡温度,Cu的去除率降低而Zn的增加;随着提取次数的增加,Cu和Zn的去除率均增加,Cu的去除以第1次提取为主,Zn的4次提取均有较好效果,用3%的皂角苷溶液连续4次提取后,Cu和Zn的累积去除率最大分别达到32.61%和39.32%;增加振荡时间、提高振荡温度、增加提取次数以及降低皂角苷溶液pH均能降低浸出液的pH。经过3%的皂角苷溶液的提取,污泥中各形态Cu的含量均减少,其中66%以上的酸溶态被去除;而Zn的酸溶态含量减少不明显,可还原态、可氧化态和残渣态均有所降低,但以残渣态的减少为主。     

Arsenic Removal from Water by Iron-Modified Bamboo Charcoal

The effectiveness of a novel and low-cost adsorbent, iron-modified bamboo charcoal (BC-Fe), for arsenic removal from aqueous systems was evaluated in this study. The BC-Fe was synthesized by loading iron onto bamboo charcoal via soaking in a ferric salt solution. The BC-Fe possessed a porous structure with a surface area of 277.895 m²/g. The adsorption characteristics of arsenic onto BC-Fe were further investigated at various pHs, contact times, arsenic concentrations, and adsorbent doses in batch tests. The corresponding optimum equilibrium pH ranges for As(III) and As(V) removal were 4–5 and 3–4, respectively. The equilibrium times for As(III) and As(V) adsorption were 30 and 35.5 h, respectively. The arsenic removal was strongly dependent on the initial adsorbate concentration and adsorbent dosage. The maximum arsenic removal capacities of BC-Fe under the experimental conditions were 7.237 and 19.771 mg/g for As(III) and As(V), respectively. The pseudo-second-order kinetic model and Freundlich isotherm explained the kinetic and equilibrium of both the As(III) and As(V) adsorbent processes, respectively. Based on these results, the BC-Fe developed in this study is a promising material for the treatment of arsenic-contaminated water.     

Arsenic extractability with phosphate and citrate from peat collected in Bangladesh

Abstract

Arsenic extractability with various solutions from peat samples collected in Deuli Village, Bangladesh, was investigated to reveal the leaching mechanisms of arsenic in sediments to groundwater. Peat layer was distributed at the depth of 7–8 m in the sediments, and the peat samples collected and used in this experiment contained 137 mg kg^?1 arsenic. A wide range of pH values and concentrations of phosphoric acid and trisodium citrate was used for the extracting solutions. Arsenic extractability with Hel (25–1,000 mM) or NaOH (20–500 mM) at different pH values ranged for 30 to 60% in strong acid or alkali, while was less than 0.1% in the neutral pH range against the total content. Extraction of arsenic with 100 mM phosphoric acid and 100 mM trisodium citrate was about 20% even at a neutral pH, while the extractability with these solutions at concentrations below 1 mM was similar to that with distilled water. These results suggest that competitive exchange and / or chelating action are / is the prior possible mechanism for arsenic leaching from peat. However, the experimental conditions such as pH value or phosphate anion concentration differed from those in the study area where the pH of groundwater was almost neutral and the phosphate anion concentration was only 0.01 mM. Moreover, since the experiment was conducted using a batch system under oxidative conditions and not using a continuous flow system, the importance of the oxidation-reduction potential in the sediments and organic matter content in the groundwater should be investigated.