Effects of water chemistry on arsenic removal from drinking water by electrocoagulation

Exposure to arsenic through drinking water poses a threat to human health. Electrocoagulation is a water treatment technology that involves electrolytic oxidation of anode materials and in-situ generation of coagulant. The electrochemical generation of coagulant is an alternative to using chemical coagulants, and the process can also oxidize As(III) to As(V). Batch electrocoagulation experiments were performed in the laboratory using iron electrodes. The experiments quantified the effects of pH, initial arsenic concentration and oxidation state, and concentrations of dissolved phosphate, silica and sulfate on the rate and extent of arsenic removal. The iron generated during electrocoagulation precipitated as lepidocrocite (γ-FeOOH), except when dissolved silica was present, and arsenic was removed by adsorption to the lepidocrocite. Arsenic removal was slower at higher pH. When solutions initially contained As(III), a portion of the As(III) was oxidized to As(V) during electrocoagulation. As(V) removal was faster than As(III) removal. The presence of 1 and 4 mg/L phosphate inhibited arsenic removal, while the presence of 5 and 20 mg/L silica or 10 and 50 mg/L sulfate had no significant effect on arsenic removal. For most conditions examined in this study, over 99.9% arsenic removal efficiency was achieved. Electrocoagulation was also highly effective at removing arsenic from drinking water in field trials conducted in a village in Eastern India. By using operation times long enough to produce sufficient iron oxide for removal of both phosphate and arsenate, the performance of the systems in field trials was not inhibited by high phosphate concentrations.     

Removal of arsenic(V) from spent ion exchange brine using a new class of starch-bridged magnetite nanoparticles

Ion exchange (IX) is considered by US EPA as one of the best available technologies for removing arsenic from drinking water. However, typical IX processes will generate large volumes of arsenic-laden regenerant brine that requires costly further handling and disposal. This study aimed to develop an engineered strategy to minimize the production and arsenic leachability of the process waste residual. We prepared and tested a new class of starch-bridged magnetite nanoparticles for removal of arsenate from spent IX brine. A low-cost, “green” starch at 0.049% (w/w) was used as a stabilizer to prevent the nanoparticles from agglomerating and as a bridging agent allowing the nanoparticles to flocculate and precipitate while maintaining their high arsenic sorption capacity. When applied to a simulated spent IX brine containing 300 mg/L As and 6% (w/w) NaCl, nearly 100% removal of arsenic was achieved within 1 h using the starch-bridged nanoparticles at an Fe-to-As molar ratio of 7.6, compared to only 20% removal when bare magnetite particles were used. Increasing NaCl in the brine from 0 to 10% (w/w) had little effect on the arsenic sorption capacity. Maximum uptake was observed within a pH range of 4-6. The Langmuir capacity coefficient was determined to be 248 mg/g at pH 5.0. The final treatment sludge was able to pass the TCLP (Toxicity Characteristic Leaching Procedure) based leachability of 5 mg/L as As.     

Immobilization of mercury in sediment using stabilized iron sulfide nanoparticles

Mercury (Hg) immobilization using stabilized iron sulfide (FeS) nanoparticles was investigated through a series of batch and column experiments. The nanoparticles were prepared using a low-cost, food-grade cellulose (sodium carboxymethyl cellulose, CMC) as the stabilizer. The hydrodynamic diameter of fresh FeS–CMC nanoparticles was measured to be 38.5 ± 5.4 nm. Batch tests showed that the nanoparticles can effectively immobilize Hg in a clay loam sediment. The Hg distribution coefficient for the nanoparticles was determined to be 8930 ± 1480 L/g, which is >4 orders of magnitude greater than for the sediment. When the Hg-laden sediment was treated at an FeS-to-Hg molar ratio of 26.5, the Hg concentration leached into water was reduced by 97% and the TCLP (toxicity characteristic leaching procedure) leachability of Hg was reduced by 99%. Column tests showed that water-leachable mercury from the sediment containing 3120 mg/L Hg was reduced by 67% and the TCLP leachability by >77% when the sediment was treated with 67 pore volumes (PVs) of a 0.5 g/L FeS nanoparticle suspension. Column tests proved that the stabilized nanoparticles were highly mobile in the sediment and full breakthrough of the nanoparticles occurred at ∼18 PVs.     

Effect of pH, competitive anions and NOM on the leaching of arsenic from solid residuals

Implementation of the new arsenic MCL in 2006 will lead to the generation of an estimated 6 million pounds of arsenic-bearing solid residuals (ABSRs) every year, which will be disposed predominantly in non-hazardous landfills. The Toxicity Characteristic Leaching Procedure (TCLP) is typically used to assess whether a waste is hazardous and most solid residuals pass the TCLP. However, recent research shows the TCLP significantly underestimates arsenic mobilization in landfills. A variety of compositional dissimilarities between landfill leachates and the TCLP extractant solution likely play a role. Among the abiotic factors likely to play a key role in arsenic remobilization/leaching from solid sorbents are pH, and the concentrations of natural organic matter (NOM) and anions like phosphate, bicarbonate, sulfate and silicate. This study evaluates the desorption of arsenic from actual treatment sorbents, activated alumina (AA) and granular ferric hydroxide (GFH), which are representative of those predicted for use in arsenic removal processes, and as a function of the specific range of pH and concentrations of the competitive anions and NOM found in landfills. The influence of pH is much more significant than that of competing anions or NOM. An increase in one unit of pH may increase the fraction of arsenic leached by 3-4 times. NOM and phosphate replace arsenic from sorbent surface sites up to three orders of magnitude more than bicarbonate, sulfate and silicate, on a per mole basis. Effects of anions are neither additive nor purely competitive. Leaching tests, which compare the fraction of arsenic mobilized by the TCLP vis-a-vis an actual or more realistic synthetic landfill leachate, indicate that higher pH, and greater concentrations of anions and NOM are all factors, but of varying significance, in causing higher extraction in landfill and synthetic leachates than the TCLP.     

Arsenic Removal from Contaminated Groundwater by Zero Valent Iron: a Mechanistic and Long-Term Performance Study

The present study investigated the application of zero valent iron to remediate the arsenic in naturally contaminated groundwater. A performance evaluation was conducted in the laboratory on groundwater contaminated with artificial arsenic using sodium arsenate (Na₂HAsO₄.7H₂O) to simulate the arsenic concentration in the groundwater. Batch and column experiments were performed to evaluate the arsenic removal capacity by zero valent iron and the removal mechanism. The flow rate (up-flow mode) was maintained for 180 days in each column. The results from both the batch and the column experiments showed that more than 99% of the arsenic was removed successfully. In the column experiments, the arsenic was efficiently removed and the arsenic concentration in the treated water decreased to below the limit of 10 μg /L (WHO's standard) even when the columns were packed with only 25% ZVI by volume. We used SEM and XRD to characterize the surface morphology and the corrosion layer which formed on pristine ZVI and arsenic-treated ZVI to elucidate the arsenic removal mechanism. XRD and SEM results revealed that ZVI gradually converted to a magnetite/maghemite corrosion product mixed with lepidocrocite. Adsorption followed by co-precipitation was an important pathway to removing the arsenic by ZVI. Our results suggest that ZVI, combined with sand, is a suitable candidate for the ex-situ treatment of groundwater in the neutral pH range and in the presence of dissolved oxygen.     

Nitrate removal in zero-valent iron packed columns

Nitrate removal by laboratory and field continuous-flow zero-valent iron (Fe(0)) packed bed columns was evaluated for different influent water qualities (pH, dissolved oxygen (DO), nitrate concentration) and several months of operation (600-1500 bed volumes (BVs)). In contrast to previous batch experiments with Fe(0) where nitrate was stoichiometrically converted to ammonium, only 70% of the applied nitrogen was recovered as nitrate, ammonium, or nitrite (<0.1mg/L) during shorter-term column tests (2-20 BVs) and less than 25% of the applied nitrogen was recovered during longer-term field testing (500-1000 BVs) at elevated nitrate levels (approximately 25mg N/L). Nitrate removal was accompanied by a pH increase, DO decrease, and soluble iron increase. During longer-term operation (500-1500 BVs) iron and calcium precipitates were observed, by SEM and EDX analyses, to form in the packed columns. Precipitation led to cementation and reduction in permeability for the Fe(0)/sand media in the packed column. Different abiotic and microbial-mediated mechanisms may be involved during shorter- and longer-term operation of Fe(0) systems and the role of iron precipitates should be further evaluated.     

A mass balance approach for evaluating leachable arsenic and chromium from an in-service CCA-treated wood structure

Many existing residential wood structures, such as playsets and decks, have been treated with chromated copper arsenate (CCA). This preservative chemical can be released from these structures incrementally over time through contact with rainfall. The objective of this study was to evaluate the levels of arsenic and chromium leached from an in-service CCA-treated deck exposed to rainfall, as well as their possible impacts on soils and shallow groundwater. Two monitoring stations, one containing a CCA-treated deck and the other containing an untreated deck as a control, were constructed outside for this study. Rainfall, runoff water from the decks, soils below the decks, and infiltrated water through 0.7-m depth of soil were monitored for arsenic and chromium over a period of 3 years. The concentration of the CCA-treated deck runoff for arsenic (0.114-4.66 mg/L) and chromium (0.008-0.470 mg/L) were significantly (p<0.001) higher than the untreated deck runoff (< or =0.002 mg/L for both). During the 3-year monitoring period, 13% of the arsenic and 1.4% of the chromium were leached from the amount initially present in the CCA-treated wood. Arsenic levels (<0.1-46 mg/kg) in soils under the CCA-treated deck were significantly (p<0.001) higher than under the untreated deck (<0.1-2.7 mg/kg), while chromium levels were statistically the same below the two decks (2.4-9.6 mg/kg). Approximately 94% of the arsenic from the runoff was absorbed in the soils below the CCA-treated deck; the upper 2.5 cm of the soils captured 42% of the total. The infiltrated water concentrations for arsenic (<0.001-0.085 mg/L) and chromium (<0.001-0.010 mg/L) below the CCA-treated deck were both significantly (p<0.001) higher than below the untreated deck (< or =0.006 mg/L). The amounts of arsenic found in the infiltrated water below the CCA-treated deck represented 6% of total arsenic leached and less than 0.7% of the initial mass in the wood. The study demonstrated that exposure of a CCA-treated deck to rainfall resulted in elevated arsenic concentrations in both runoff and soil. Although only a relatively small fraction of the initial arsenic from the wood was found to infiltrate through the soil, these impacts were significant and caused the infiltrated water to exceed drinking water standards. The study suggests that potential exposures to arsenic exist indirectly through an environment that is contaminated with arsenic leached from in-service CCA-treated wood.     

铁砷复合污染下铁砷比对除砷效果的影响研究

针对铁砷复合污染型地下水,以原水铁砷比作为控制参数,通过烧杯试验研究了曝气接触氧化除铁工艺的除砷效果。结果表明,当初始砷含量分别为100,200,300和400μg/L时,原水铁砷比分别为35∶1,50∶1,52∶1和55∶1,能达到除铁效果且同时满足出水砷含量小于10μg/L的限值要求;根据氢氧化铁对砷的吸附机理,利用Freundlich吸附等温式建立了铁砷比与残余砷含量的数学模型,试验数据拟合结果与模型相吻合。此外,采用曝气氧化工艺处理铁砷复合污染地下水时,可以通过投加二价铁盐控制原水铁砷比,以实现同时去除铁砷的目的。     

Subsurface iron and arsenic removal for shallow tube well drinking water supply in rural Bangladesh

Subsurface iron and arsenic removal has the potential to be a cost-effective technology to provide safe drinking water in rural decentralized applications, using existing shallow tube wells. A community-scale test facility in Bangladesh was constructed for injection of aerated water (∼1 m³) into an anoxic aquifer with elevated iron (0.27 mmol L⁻¹) and arsenic (0.27 μmol L⁻¹) concentrations. The injection (oxidation) and abstraction (adsorption) cycles were monitored at the test facility and simultaneously simulated in the laboratory with anoxic column experiments.Dimensionless retardation factors (R) were determined to represent the delayed arrival of iron or arsenic in the well compared to the original groundwater. At the test facility the iron removal efficacies increased after every injection-abstraction cycle, with retardation factors (R_Fe) up to 17. These high removal efficacies could not be explained by the theory of adsorptive-catalytic oxidation, and therefore other ((a)biotic or transport) processes have contributed to the system’s efficacy. This finding was confirmed in the anoxic column experiments, since the mechanism of adsorptive-catalytic oxidation dominated in the columns and iron removal efficacies did not increase with every cycle (stable at R_Fe = ∼8). R_As did not increase after multiple cycles, it remained stable around 2, illustrating that the process which is responsible for the effective iron removal did not promote the co-removal of arsenic. The columns showed that subsurface arsenic removal was an adsorptive process and only the freshly oxidized adsorbed iron was available for the co-adsorption of arsenic. This indicates that arsenic adsorption during subsurface treatment is controlled by the amount of adsorbed iron that is oxidized, and not by the amount of removed iron. For operational purposes this is an important finding, since apparently the oxygen concentration of the injection water does not control the subsurface arsenic removal, but rather the injection volume. Additionally, no relation has been observed in this study between the amount of removed arsenic at different molar Fe:As ratios (28, 63, and 103) of the groundwater. It is proposed that the removal of arsenic was limited by the presence of other anions, such as phosphate, competing for the same adsorption sites.     

Effect of interferences on the breakthrough of arsenic: rapid small scale column tests

The influences of three important interferences (silica, phosphate, and vanadate) and the effect of different pH levels and initial arsenate concentrations on the breakthrough of arsenic in adsorptive media columns were examined by using the Rapid Small Scale Column Test with a 3⁵⁻² fractional factorial design. Three commercially available adsorbents used for arsenic removal (E33, GFH and Metsorb) were tested. Results indicated that GFH was more susceptible to water quality changes than Metsorb and E33 under conditions tested. GFH also adsorbed more anions than the other two media. The pH was the factor that had the most impact on the performance of the columns, followed by arsenic concentration and silica concentration. Lowering pH from 8.3 to 7.0 resulted in an increase of the mean bed volume treated until 10 μg/L arsenic breakthrough by 40, 12 and 18 thousands BV treated by GFH, E33 and Metsorb columns, respectively. However, at high silica concentration, lowering pH did not increase the performance of the media. GFH and Metsorb were more sensitive to changes in arsenic concentration at low pH than at high pH. Although vanadium and phosphate were previously reported to reduce arsenic adsorption in batch tests, in column mode with the presence of competitors, their effect was insignificant compared to that of pH, arsenic or silica under the conditions used in this study.     

高铁矿井水回用处理工艺研究

针对韩桥矿高铁矿井水质,考察了混凝＋沉淀＋过滤回用处理工艺除铁的最佳运行条件。结果表明,以聚合氯化铝（PAC）作为混凝剂,最佳投药量为80mg／L时,铁的去除率可达到91％;选择天然锰砂作为滤料,滤速在8～10m／h,进水pH值高于6．05,滤层高度大于75cm时,锰砂除铁效果较好,出水铁含量在0．1mg／L以下,可以满足《城市污水再生利用工业回用水水质》（GB／T19923--2005）中的限值要求。     

Batch-Mixed Iron Treatment of High Arsenic Waters

This paper develops batch-mixed treatment with zero-valent iron as a point-of-use technology, appropriate for arsenic removal from water stored within rural homes in Bangladesh and West Bengal, India, where arsenic poisoning has affected an estimated 20 million people. Batch tests with iron yielded the following results: (1) High arsenic removal (>93%) was achieved from highly arsenated waters (2000 μg/L) over short contact times (0.5–3 h) with iron filings added at doses ranging from 2500 to 625 mg/L; (2) Most rapid arsenic removal was observed in head-space free systems with sulphates present in solution, while phosphate buffers were observed to inhibit arsenic removal by iron; (3) The arsenic removed from water was found to be strongly bound to the elemental iron filings, such that the treated water could be decanted and iron could be reused at least 100 times; (4) Some iron dissolved into water over the contact period, at concentrations ranging from 100 to 300 μg/L, which are within safe drinking water limits. These results indicate that, with appropriate assessment of water chemistry in the affected region, treatment with metallic iron followed by simple decantation can be used as a practical, in-home, point-of-use technique for reducing human exposure to arsenic in drinking water.     

Effects of water chemistry and flow rate on arsenate removal by adsorption to an iron oxide-based sorbent

Arsenate removal from water using an iron oxide-based sorbent was investigated to determine the optimal operating conditions and the influence of water composition on treatment efficiency. The novel sorbent with a high surface area was studied in flow-through column experiments conducted at different flow rates to quantify the effect of empty bed contact time (EBCT) on treatment performance. Arsenic removal efficiency declined with decreasing EBCT. Arsenic breakthrough curves at different EBCT values were successfully simulated with a pore and surface diffusion model (PSDM). Surface diffusion was the dominant intraparticle mass transfer process. The effect of water composition on arsenic removal efficiency was evaluated by conducting experiments with ultrapure water, ultrapure water with either phosphate or silica, and a synthetic groundwater that contained both phosphate and silica. Silica was more inhibitory than phosphate, and the silica in synthetic groundwater controlled the arsenic removal efficiency.     

Arsenic removal from aqueous solutions by adsorption on magnetite nanoparticles

Magnetite nanoparticles were used to treat arsenic‐contaminated water. Because of their large surface area, these particles have an affinity for heavy metals by adsorbing them from a liquid phase. The results of the study showed that the maximum arsenic adsorption occurred at pH 2, with a value of approximately 3.70 mg/g for both As(III) and As(V) when the initial concentration of both arsenic species was maintained at 2 mg/L. The study showed that, apart from pH, the removal of arsenic from contaminated water also depends on the contact time, the initial concentration of arsenic, the phosphate concentration in the water and the adsorbent concentration. The results suggest that arsenic adsorption involved the formation of weak arsenic–iron oxide complexes at the magnetite surface. At a fixed adsorbent (magnetite nanoparticles) concentration of 0.4 g/L, percent arsenic removal decreased with increasing phosphate concentration. Magnetite nanoparticles removed <50% of arsenic from water containing >6 mg/L phosphate. In this case, an optimum design for achieving high arsenic removal by magnetite nanoparticles may be required.     

Oxidation and removal of arsenic (III) from aerated groundwater by filtration through sand and zero-valent iron

Removing arsenic from contaminated groundwater in Bangladesh is challenging due to high concentrations of As(III), phosphate and silicate. Application of zero-valent iron as a promising removal method was investigated in detail with synthetic groundwater containing 500 microg/L As(III), 2-3mg/L P, 20mg/L Si, 8.2mM HCO3-, 2.5mM Ca2+, 1.6mM Mg2+ and pH 7.0. In a series of experiments, 1L was repeatedly passed through a mixture of 1.5 g iron filings and 3-4 g quartz sand in a vertical glass column (10mm diameter), allowing the water to re-aerate between each filtration. At a flow rate of 1L/h, up to 8 mg/L dissolved Fe(II) was released. During the subsequent oxidation of Fe(II) by dissolved oxygen, As(III) was partially oxidized and As(V) sorbed on the forming hydrous ferric oxides (HFO). HFO was retained in the next filtration step and was removed by shaking of the sand-iron mixture with water. Rapid phosphate removal provided optimal conditions for the sorption of As(V). Four filtrations lead to almost complete As(III) oxidation and removal of As(tot) to below 50 microg/L. In a prototype treatment with a succession of four filters, each containing 1.5 g iron and 60 g sand, 36 L could be treated to below 50 microg/L in one continuous filtration, without an added oxidant.     

A novel two-step coprecipitation process using Fe(III) and Al(III) for the removal and immobilization of arsenate from acidic aqueous solution

Lime neutralization and coprecipitation of arsenate with iron is widely practiced for the removal and immobilization of arsenic from mineral processing effluents. However, the stability of the generated iron-arsenate coprecipitate is still of concern. In this work, we developed a two-step coprecipitation process involving the use of iron and aluminum and tested the stability of the resultant coprecipitates. The two-step Fe-As-Fe or Fe-As-Al coprecipitation process involved an initial Fe/As = 2 coprecipitation at pH4 to remove arsenic from water down to 0.25 mg/L, followed by introduction of iron or aluminum (Fe/As = 2, Al/As = 1.5 or 2). The two-step coprecipitates showed higher stability than traditional Fe/As = 4 coprecipitate under both oxic and anoxic conditions. Leaching stability was enhanced when aluminum was applied in the second step. The use of aluminum in the second step also inhibited microbial mediated arsenate reduction and arsenic remobilization. The results suggest that the two-step coprecipitation process is superior to conventional coprecipitation methods with respect to the stability of the generated arsenic-bearing solid waste. The use of Al in the second step is better than Fe to enhance the stability. This work may have important implications to the development of new technologies for efficient arsenic removal from hydrometallurgical solutions and safe disposal in both oxic and anoxic environment.     

预氧化-化学沉淀法去除水中砷的试验研究

研究了预氧化-化学沉淀法对水中砷的去除效果及其影响因素。结果表明,原水砷质量浓度为5倍标准限值时,在NaClO预氧化条件下,投加8 mg/L的聚合氯化铝可使砷去除率达到84%,且出水砷含量可以满足《生活饮用水卫生标准》(GB 5749—2006)限值要求;在KMnO4预氧化条件下,投加8 mg/L的聚合氯化铝可使砷去除率达到90%,且出水砷含量满足标准限值要求,而投加8 mg/L的聚合硫酸铁可使出水砷含量降至18.72μg/L,无法满足标准限值要求;采用聚合氯化铝作为混凝剂时的除砷效果优于聚合硫酸铁,以KMnO4作为预氧化剂时的除砷效果优于NaClO。     

Effect of silica and pH on arsenic uptake by resin/iron oxide hybrid media

The recently imposed maximum contaminant level (MCL) of 10microg/L for arsenic has necessitated many water providers to implement efficient treatment systems to reduce the arsenic content in potable water supplies across the United States. A popular, cost-effective solution is to adsorb the arsenic onto hydrous iron oxide-based granular media. Hybrid media, consisting of hydrous iron oxides impregnated into a polymeric substrate in order to improve mechanical stability, have also been developed. The effective operational bed life of these adsorptive media is strongly dependent on the chemistry of the water being treated. High levels of silica combined with pH values greater than 8 have been shown to have a detrimental effect on the arsenic removal efficiency of all adsorptive media. Batch arsenate (arsenic(V)) adsorption experiments were performed to evaluate the effect of pH and silica on the static arsenic capacity of two iron oxide-based hybrid media, ArsenX(np) (a commercially available media) and npRio (a developmental media). From the data obtained, it was evident that the presence of increased levels of silica enhanced the detrimental effect of elevated pH on arsenic capacity, being most noticeable at pH 8 and above. In a solution containing 30ppm of SiO(2), a decrease in arsenic capacity as high as 71.8% was observed when the pH was increased from 7 to 9. Reducing the pH of the water prior to treatment may therefore be an economic option for improving media performance in drinking waters containing high concentrations of silica.     

Characteristics and stabilities of residues from the Wheal Jane constructed wetlands

The characteristics and solubilities of residues formed during effluent treatment at the Wheal Jane constructed wetland facility in Cornwall, UK are described. The constructed wetland treats an acidic (pH 3 to 4) mine discharge enriched in iron (<100 mg/l), zinc (<80 mg/l), manganese (<20 mg/l) and arsenic (<2 mg/l). The processing strategy adopted at the site is to remove elements selectively from the mine water in distinct unit process operations and concentrate the iron and base metals into residues within the circuits. Solubility tests conducted on the materials from the aerobic and anaerobic cells attempt to simulate future possible leaching processes within landfill sites used for ultimate disposal. The tests used in the evaluation included TCLP, MARG, washing and column leaching. The aerobic solids overall have very low solubilities which are well below the TCLP thresholds for As, Cd and Pb. The TCLP results for these toxic elements are supported by the washing and column test work. The anaerobic substrates are carbonaceous in nature, composed of straw, wood pulp and manure. They contain minimal amounts of toxic compounds and are considered to be less of a disposal problem. Ochre precipitated at a pH of 3-4 in the aerobic cells contains significant concentrations of arsenic (>0.1%). This element is carcinogenic, and such residues are classified as hazardous waste under current UK landfill regulations. This classification does not take into consideration the solubility of the material but is based solely on the bulk chemical composition.     

地下水生物除铁效果及其动力学研究

采用生物滤柱进行了地下水除铁的试验研究。当原水中Fe^2＋的质量浓度为4．3mg／L,pH值为6．4～6．6,水温为23～25℃,DO为1．5mg／L,滤速为8m／h时,出水中Fe^2＋〈0．1mg,／L。通过灭菌试验得出,滤柱对铁的去除主要通过生物氧化完成,而非物理化学作用。通过分析不同高度滤层水中铁的含量研究了生物氧化除铁动力学规律,得出铁含量与空床接触时间之间的函数关系。