Interaction of organic acids and pH on multi-heavy metal extraction from alkaline and acid mine soils

Vegetation at mining sites can produce increased heavy metal leaching by the organic acids and protons originating from root secretion and litter degradation. Batch experiments were conducted to investigate the effects of organic acids and pH on the extraction of Pb, Cd, Zn and Cu from an alkaline mine soil (sampled from a mining site of Chenzhou City, Hunan Province) and an acid mine soil (sampled from a mining site of Daxin county, Guangxi Province). The results showed that in the presence of organic acids (acetic, oxalic, malic, fumaric, tartaric and citric acids) at pH 7, the extraction of Pb, Cd, Zn and Cu from the acid mine soil was much higher than that from the alkaline mine soil, in which only citric acid with higher concentration was capable of extracting some heavy metals. Citric acid had the strongest ability in extracting heavy metals, followed by oxalic acid. Heavy metal extraction dramatically decreased with increasing pH. Moreover, at low pH, oxalic acid promoted the risk of Cu leaching; at high pH, the leaching of Pb, Zn, Cd and Cu was enhanced by both oxalic and citric acids. This indicated that those plants, which can produce substantial citric acid or oxalic acid by root secretion and litter degradation, should not be selected for the revegetation of mining sites.     

Enhanced microbial reduction of Cr(VI) and U(VI) by different natural organic matter fractions

Although direct microbial reduction of Cr(VI) and U(VI) is known, few studies have examined the kinetics and the underlying mechanisms of the reduction of these contaminants by different natural organic matter (NOM) fractions in the presence or absence of microorganisms. In this study, NOM was found to chemically reduce Cr(VI) at pH 3, but the reduction rates were negligible at pH ∼7. The abiotic reduction of U(VI) by NOM was not observed, possibly because of the presence of small amounts of nitrate in the reactant solution. However, all NOM fractions, particularly the soil humic acid (HA), enhanced the bioreduction of Cr(VI) or U(VI) in the presence of Shewanella putrefaciens CN32. The reduction rates varied greatly among NOM fractions with different chemical and structural properties: the polyphenolic-rich NOM-PP fraction appeared to be the most reactive in abiotically reducing Cr(VI) at a low pH, but soil HA was more effective in mediating the microbial reduction of Cr(VI) and U(VI) under anaerobic, circumneutral pH conditions. These observations are attributed to an increased solubility and conformational changes of the soil HA with pH and, more importantly, its relatively high contents of polycondensed and conjugated aromatic organic moieties. An important implication of this study is that, depending on chemical and structural properties, different NOM components may play different roles in enhancing the bioreduction of Cr(VI) and U(VI) by microorganisms. Polycondensed aromatic humic materials may be particularly useful in mediating the bioreduction and rapid immobilization of these contaminant metals in soil.     

Adsorption Behavior of Hexavalent Chromium in Vadose Zone

Adsorption behavior of Cr (VI) in vadose zone, which is silty clay and clayey soil, was studied through kinetics experiments, isothermal adsorption experiments under various conditions, including different ph, temperature and organic contents. The results from kinetics experiments showed that the sorption progress of Cr (VI) has clear features in different stages, and adsorption equilibrium showed at 30 min, the adsorption rate of silty clay and clayey soil were 60%. The isothermal adsorption curve of Cr (VI) fitted closely with Freundlich equation model. When pH is 3-5 a plateau were seen, thereafter with increase in pH the adsorption rate of Cr (VI) dropped sharply and the minimum achieved at pH 10, the adsorption rate were only 35%. Adsorption rate of Cr (VI) increased gradually with the increase of temperature, the temperature of vadose zone is 14.7 ℃, according to the experimental results, the adsorption rate of Cr (VI) is about 40%. The use of organics represents an important contribution to the sorption of Cr (VI), sorption rate up to 100% when 30% of organic content. These studies will provide basis for manager to minimize the impacts, and provide basic data for pollution prevention and remediation of vadose zone.     

考虑温度电动淋洗法去除污染软土中铬试验研究

针对铬污染软土,自行设计了电动淋洗试验装置,开展了铬污染软土室内土柱淋洗试验,分析了试验过程中电流、电解质溶液pH的变化以及淋洗液种类、外加电压、温度对重金属铬去除特性的影响。结果表明:电动淋洗试验可以有效去除土壤中的重金属铬;相对于单一淋洗试验,用10 V电压强化淋洗试验显著提高了重金属铬去除效果,淋洗液为十二烷基苯磺酸钠（SDS）时Cr（VI）和Cr（总）去除效率是单一淋洗法的2.79,3.12倍。当电压为10 V,温度升高至45 ℃时,淋洗液为柠檬酸（CA）和草酸（OA）的各组试验表明Cr（Ⅵ）与Cr（总）去除率均相应提高;而淋洗液为十二烷基苯磺酸钠时的试验组中Cr（VI）去除率提高了5.84%,Cr（总）去除率降低了4.25%,表明升高温度使部分Cr（VI）还原成不易迁移的Cr（III）;淋洗液为草酸的试验组中升高温度时Cr（Ⅵ）与Cr（总）去除率最高,分别达到了82.08%、77.57%,分别相应提高了27.65%、26.01%。电动淋洗试验后,铬污染软土土粒结构变得更紧密,土粒之间的孔隙减小且被填充的更加密实。     

Characterization and evaluation of a tropical peat for the removal of Cr(VI) from solution

Tropical peat soils present higher ash content than those generated at temperate climate areas. Therefore, this study evaluated the characteristics of a Brazilian organic soil (OS), commercialized as peat, as well as its capacity in removing Cr(VI) from contaminated waters. The OS is composed of 35.5 wt% of organic matter and 56 wt% of inorganic fraction (ash), which is formed by minerals and phytoliths rich in silica (29.2 wt%) and alumina (23.6 wt%). The Cr(VI) removal tests were carried out in batch and column systems using OS and solutions of Cr(VI) prepared with distilled water and groundwater. Batch tests revealed that the organic substances in the OS caused the reduction of Cr(VI) to Cr(III), with an efficiency depending on solution pH. At pH 5.0 the Cr(VI) removal was 0.45 mg g^?1 in 24 h; whereas at pH 2.0, this removal increased to 1.10 mg g^?1. Since this redox reaction is very slow, the removal of Cr(VI) at pH 5.0 increased to around 2 mg g^?1 after 5 days. The removal of Cr(VI) was more effective in the column tests than in the batch test due to the greater solid/solution ratio, and their half-lives were 4.4 and 26.2 h, respectively. Chemical analysis indicated that Cr(VI) was reduced by the humic substances of OS, followed by the precipitation and/or adsorption of Cr(III) into the organic and inorganic components, as anatase. The presence of Cr(III) increased the stability of anatase structure, avoiding its transformation into rutile, even after being heated at 800 °C/2 h.     

低分子有机酸对聚合羟基铁铝-蒙脱石复合体吸附磷的影响

在一定条件下,利用钠基化蒙脱石(Na-Mt)合成聚合羟基铁铝-蒙脱石复合体(HyFeAl-Mt),M3+/Mt比为10mmol/g,Fe3+/Al3+摩尔比分别为0.1、0.2和0.5。研究了酸性和弱酸性条件下低分子有机酸(以柠檬酸、草酸和苹果酸为代表)及其添加顺序对HyFeAl-Mt复合体吸附磷的影响。结果表明,低分子有机酸对HyFeAl-Mt复合体吸附磷有抑制作用,且随着低分子有机酸浓度的增加愈加明显,酸性条件下的抑制作用比弱酸性条件下明显;低分子有机酸羧基个数越多、分子结构越大,对HyFeAl-Mt对吸附磷的影响越大;酸性环境中低分子有机酸与磷共存时,磷吸附曲线较好地符合Freundlich方程,且其对磷在HyFeAl-Mt0.2上的最大吸附量影响较小;磷与低分子有机酸的添加顺序不同也影响HyFeAl-Mt复合体对磷的吸附。     

Comparison of fast elimination of Cr(VI) by alumina nanofiber and alumina nanoporous

The goal of this study is examination of the mixture between adsorption and permeation process for removing chromium (VI) from the water. Two types of supported membranes are developed: The first one which was made by sol–gel method is called nanoporous and the second one which was made by electrospinning is called nanofiber. The sorption capacity of nanoporous and nanofiber is examined in single batch experiments at various pH values, and it is found that maximum chromium removal is observed for both nanoporous and nanofiber at pH 3.5. Adsorption studies illustrated that the Cr(VI) adsorption onto alumina nanoporous and nanofiber is affected by changes in pH, contact time, dosage of adsorbent, concentration of chromium and solution volume. Langmuir and Freundlich isotherms can be used to explain the adsorption equilibria of Cr(VI) onto alumina nanoporous and nanofiber. It was found that balance adsorption data adequate Langmuir isotherm more than Freundlich model. Adsorption kinetics was found to be fitted to pseudo-second order and Weber and Morris model. The output of multiple linear regressions was run for the second-order response surface model implied that the linear agents of pH, sorbent dosage and Cr(VI) concentration are more significant factors. Manufacturing electrospun alumina nanofiber and sol–gel nanoporous with these cheap materials, renewable and fast methods are so important although the removal percentage is significant.     

Adsorption of heavy metals in glacial till soil

The charged sites on soil particles are important for the retention/adsorption of metals. Metallic counterions can neutralize the intrinsic charges on the surfaces of soil particles by forming complexes. In this study, efforts have been made to determine the effect of surface potential, pH, and ionic strength on the adsorption of four metal ions, hexavalent chromium Cr(VI), trivalent chromium Cr(III), nickel Ni(II) and cadmium Cd(II), in glacial till soil. Batch tests were performed to determine the effect of pH (2–12) and ionic strength (0.001–0.1 M KCl) on zeta potential of the glacial till soil. The point of zero charge (pH _PZC ) of glacial till was found to be 7.0±2.5. Surface charge experiments revealed the high buffering capacity of the glacial till. Batch adsorption experiments were conducted at natural pH (8.2) using various concentrations of selected metals. The adsorption data was described by the Freundlich adsorption model. Overall glacial till shows lower adsorption affinity to Cr(VI) as compared to cationic metals, Cr(III), Ni(II) and Cd(II).     

Kinetics and equilibrium study of chromium adsorption on zeoliteNaX

This study aims to report Batch adsorption study of hexavalent chromium, Cr (VI) on zeoliteNaX. Kinetics of Cr (VI) adsorption and adsorption isotherms were determined by varying operating parameters such as pH, initial concentration, temperature and contact time. ZeoliteNaX was found to remove Cr (VI) in acidic solutions down to ppm level at pH of about 4. Removal rate of Cr (VI) was found to decrease as pH rises above 4.0. Langmuir, Freundlich, Temkin and Redlich-Peterson models were applied to adsorption equilibrium data to find the best amongst these models. Langmuir model with R² = 0.9711 best fits the adsorption data. The kinetics of adsorption was found to follow the first order reversible reaction. The separation parameter, R_L values of less than 1.0 i.e., 0.7369, 0.5834 and 0.4828 corresponding to initial concentrations of 10, 20 and 30 mg/L, respectively indicated that adsorption of Cr (VI) on zeoliteNaX is favoured. The estimated values of thermodynamic parameters such as heat of adsorption and standard gibbs free energy confirmed the exothermic nature of adsorption of Cr (VI) on zeoliteNaX.     

A study on the adsorption of chromium on laterite from Guizhou Province,China

The adsorption behaviors of Cr(VI) on laterite from Guizhou Province were studied in this paper, and the adsorption mechanism was discussed as well. Results showed that different mineral compositions in the laterite would cause differences in the capacity of laterite to absorb Cr(VI). Gibbsite, iron oxide minerals and non-crystalloids are the main contributors to enhancing the capacity of laterite to absorb Cr(VI). The pH of the solution is an important factor affecting the adsorption of Cr(VI) on laterite. Acidic environment (pH=2–5) is favorable to the adsorption of Cr(VI). The amount of adsorbed Cr(VI) decreases with increasing pH of the solution. With increasing initial concentrations of Cr(VI), the amount of adsorbed Cr(VI) increases first, and then decreases. The optimal adsorption concentration of Cr(VI) on laterite is 250 μg/mL. The adsorption of Cr(VI) on laterite is a rapid process, about 80% Cr(VI) will be adsorbed within 2 hours. And the adsorption of Cr(VI) on kaolinite is a slow process.     

深层温压条件下有机酸热稳定性模拟研究

深层、超深层中有机酸的分布特征及其热稳定性对储层物性有着重要影响。采用高温高压水—岩模拟装置对储层中典型一元和二元有机酸（乙酸和乙二酸）的分解反应进行了模拟,对动力学参数进行了计算,并分析其影响因素。结果显示:乙二酸比乙酸更容易分解,且分解反应的速率更高;乙酸和乙二酸分解反应的起始温度分别为230 ℃和180 ℃,其反应速率随着温度的升高而急剧增加。反应体系中高pH值和钾长石的存在明显提高了乙酸和乙二酸分解反应的反应速率,但对起始温度影响较小。高流体压力和静岩压力均会抑制乙酸和乙二酸的分解,在提高有机酸分解反应起始温度的同时降低分解反应速率。从地质意义上来讲,相对高压、低地温的地层环境更有利于有机酸的保存,因此具有低地温梯度的沉积盆地形成深层—超深层优质储层的可能性更高。     

黄土对Cr(III)的吸附特性及机理研究

铬是一种重金属痕量元素,人体通过食物链摄入过量的铬会在人体内富集,随之产生中毒反应。研究Cr(III)的浓度、反应时间、反应温度、pH值等因素对Cr(III)在黄土上吸附特性的影响,结果表明黄土对Cr(III)的吸附非常迅速,并且吸附量非常大,等温吸附模型Freundlich和Dubinin-Radushkevich(D-R)模型都能很好地解释Cr(III)在黄土上的吸附过程。热动力学分析表明,吸附是一个自发的过程,升温可促进吸附作用的进行。随着温度的不断升高,Cr(III)的吸附量逐渐增大。溶液的pH值是影响Cr(III)吸附效果的一个重要因素,当pH>6时,Cr(III)几乎完全被去除。利用X光衍射图谱和红外光谱分析,探讨黄土与Cr(III)的结合机制,黄土中的高岭土、石英等黏土矿物以及有机质成分对吸附过程起重要作用。     

FeSO4对铬污染土的稳定特性及风险评价试验研究

采用硫酸亚铁（FeSO4）对铬污染土进行稳定化处理。选用浸出试验、Cr(VI)残留值试验和形态提取试验,研究了粒径和有机质对铬污染土稳定特性的影响规律。试验结果表明,粒径和有机质对铬污染土稳定特性有较大影响。粒径的减小可显著降低稳定土中Cr(VI)和总Cr的浸出浓度及稳定土中Cr(VI)的含量;当污染土粒径小于2 mm时,Fe(II)/Cr(VI)摩尔比为3,稳定土中Cr(VI)和总Cr的浸出浓度分别为4.68、8.9 mg/L,均低于我国《危险废弃物鉴别标准 浸出毒性鉴别》（GB/T5085.3－2007）的限值。有机质添加量的增加可明显降低稳定土中Cr(VI)和总Cr的浸出浓度及Cr(VI)的含量。当Fe(II)/Cr(VI)摩尔比为3时,有机质的添加量为5%,稳定土中Cr(VI)的含量为28.3 mg/kg,低于我国《土壤环境质量标准》（GB15618－2008）中工业和商业用地限值（30 mg/kg）;当有机质的添加量为10%时,稳定土中Cr(VI)的含量为4.8 mg/kg,低于居住用地限值（5 mg/kg）。形态提取试验结果表明：粒径的减小可降低弱酸提取态的铬含量,增加可还原态的铬含量,而对可氧化态和残渣态的铬含量影响不大;有机质可促使弱酸提取态、可还原态的铬转化为可氧化态的铬,而残渣态的铬变化不大。稳定土中铬从活性态向较稳定态转化,是铬稳定土稳定特性和环境风险变化的根本原因。     

Comparative study on adsorption of chromium(VI) from industrial wastewater onto nature-derived adsorbents (brown coal and zeolite)

The removal of hexavalent chromium from wastewater streams has received an considerable attention in recent years, since it can cause harmful effects on the environment. Several approaches, including adsorption, are recognized to tackle this problem, but unfortunately most of these processes are impressed with practical conditions of the experiments. The main objective of this study was to recognize applicable conditions for Cr(VI) removal from an industrial drainage using nature-derived adsorbents (brown coal and modified zeolite) and to make the process more adaptive by using adsorbents conjointly. Batch experiments were carried out by agitating Cr(VI) stock solution with adsorbents at room temperature. The influence of main operating parameters was explored, and the best proportion of the adsorbents was determined. Maximum sorption of Cr(VI) onto brown coal was observed at pH = 4 by adding 60 g L^?1 adsorbent to contaminated solution. In case of using zeolite, the modification process was required, and the pH indicated a weak influence in a wide range (2–8). Optimum dosage of modified zeolite for Cr(VI) removal was 10 g L^?1. The hybrid application of adsorbents with the mass ratio of brown coal/modified zeolite at (3:1) was capable of removing more than 99% of Cr(VI) from contaminated wastewater in the natural pH range of the wastewater. The adsorption of Cr(VI) by brown coal and modified zeolite followed Langmuir and Freundlich isotherm models, respectively. Sorption of Cr(VI) onto both brown coal and modified zeolite fitted well to pseudo-second-order rate reaction.     

Effects of aqueous complexation on reductive precipitation of uranium by Shewanella putrefaciens

We have examined the effects of aqueous complexation on rates of dissimilatory reductive precipitation of uranium by Shewanella putrefaciens. Uranium(VI) was supplied as sole terminal electron acceptor to Shewanella putrefaciens (strain 200R) in defined laboratory media under strictly anaerobic conditions. Media were amended with different multidentate organic acids, and experiments were performed at different U(VI) and ligand concentrations. Organic acids used as complexing agents were oxalic, malonic, succinic, glutaric, adipic, pimelic, maleic, citric, and nitrilotriacetic acids, tiron, EDTA, and Aldrich humic acid. Reductive precipitation of U(VI), resulting in removal of insoluble amorphous UO₂ from solution, was measured as a function of time by determination of total dissolved U. Reductive precipitation was measured, rather than net U(VI) reduction to U(IV), to assess overall U removal rates from solution, which may be used to gauge the influence of chelation on microbial U mineralization. Initial linear rates of U reductive precipitation were found to correlate with stability constants of 1:1 aqueous U(VI):ligand and U(IV):ligand complexes. In the presence of strongly complexing ligands (e.g., NTA, Tiron, EDTA), UO₂ precipitation did not occur. Our results are consistent with ligand-retarded precipitation of UO₂, which is analogous to ligand-assisted solid phase dissolution but in reverse: ligand exchange with the U⁴⁺ aquo cation acts as a rate-limiting reaction moderating coordination of water molecules with U⁴⁺, which is a necessary step in UO₂ precipitation. Ligand exchange kinetics governing dissociation rates of ligands from U(VI)-organic complexes may also influence overall UO₂ production rates, although the magnitude of this effect is unclear relative to the effects of U(IV)-organic complexation. Our results indicate that natural microbial-aqueous systems containing abundant organic matter can inhibit the formation of biogenic amorphous UO₂.     

An experimental study of heavy metal attenuation and mobility in sandy loam soils

Column flow-through experiments reacting wastewater solutions with sandy loam soil samples were performed to study heavy metal attenuation by two soils with different physical and chemical properties. Reacted soil columns were leached with synthetic acid rain to study the mobility of attenuated heavy metals under leaching conditions. This study demonstrates that cation exchange, surface adsorption, chelation with solid organic material, and precipitation were the important attenuation mechanisms for the heavy metals (Cd, Cr, Cu, Mo, Ph, and Zn). Adsorption on soil hydrous oxide surfaces was the primary attenuation mechanism for Cd and Zn in both soils, and for Cu in a soil with low organic matter content. Wastewater solution pH is also an important factor that influences the retention of heavy metals. Cadmium, Cu, Cr, and Zn became mobile after prolonged application of spiked wastewater solution, either through saturation of soil adsorption sites or due to decreasing pH. Only Cr, Pb, and Mo, which are attenuated primarily through precipitation, show significant net retention by soil. Acid rain water removed heavy metals left in the column residual pore solution and weakly sorbed heavy metals in the soils, and has the ability to mobilize some strongly attenuated heavy metals, especially when the soil organic matter content is high. The results have important applications in predicting heavy metal mobility in contaminated soil, the disposal of acid mine drainage, and assessing the risks of landfall leachate leakage.     

Removal of heavy metals from a contaminated soil using organic chelating acids

Changes in heavy metal speciation and uptake by maize in a soil before and after washing with chelating organic acids, citric acid, tartaric acid and ethylenediaminetetraacetic acid were assessed. A sandy loam was collected from the vicinity of the Benue industrial layout, Makurdi, Nigeria and spiked with a quinternary mixture of nickel, copper, zinc, cadmium and lead nitrates to achieve higher levels of contamination. Batch soil washing experiments performed on 1.0 g portions of the spiked soil using 0.05 M chelating agents at a solid:liquid ratio of 1:25 showed that washing efficiencies varied in the order: ethylenediaminetetraacetic acid> citric acid> tartaric acid with metal extraction yields typically following the sequence, copper> nickel> zinc> cadmium> lead. Sequential extractions proposed by the European Communities Bureau of Reference method used to assess the redistribution of heavy metal forms in the soil showed that apparent metal mobilities were reduced upon soil washing. Citric acid removed most of the metals hitherto associated with the exchangeable and reducible fractions; tartaric acid, the exchangeable metal pools; and ethylenediaminetetraacetic acid, the non-residual metal pools. Heavy metal assay of harvested biomass of maize grown on unwashed and washed soil samples indicated that metal transfer coefficients, decreased in the order of treatment: ethylenediaminetetraacetic acid <citric acid <tartaric acid <unwashed soil. Ethylenediaminetetraacetic acid and citric acid appeared to offer greater potentials as chelating agents to use in remediating the high permeability soil. Tartaric acid, however, is recommended in events of moderate contamination.     

Effects of organic acids on dissolution of Fe and Mn from weathering coal gangue

Understanding the effects of organic acids (OA) on the transformation of Fe and Mn to surface water from the weathering coal gangue is of great benefit to risk assessment and remediation strategies for contaminated water and soil. Based on the investigation on surface water in the central coal districts of the Guizhou Province, 18 water samples were collected for heavy metal analysis. The results indicated that the pH value of surface water is low (3.11–4.92), and Fe concentration (1.31–5.55 mg L^?1) and Mn concentration (1.90–5.71 mg L^?1) were, on average, 10.86 and 34.33 times the limit of Surface Water Quality Standards, respectively. In order to evaluate the effects of the OA on the dissolution of Fe and Mn from the weathering coal gangue, column elution and batch leaching experiments were conducted. The results show that the low molecular weight of organic acids (LMWOAs, i.e., oxalic, tartaric, malic and citric acids) and fulvic acids significantly accelerated the dissolution of Fe and Mn; in addition, when the concentration of OA reached 25 mmol L^?1, the concentrations of Fe, and Mn were 1.14–67.08 and 1.11–2.32 times as high as those in 0.5 mmol L^?1 OA, respectively. Furthermore, the migration of Fe and Mn was significantly influenced by the pH and Eh, especially for Fe; the ion Mn was dissolved from the gangue more easily than the ion Fe in the column leaching, which was contrary to the results of batch leaching.     

Removal of Cr from synthetic wastewater by sorption into volcanic ash soil

The possibility of using volcanic ash soils (VAS) or Andisols as a low-cost and natural adsorbent is investigated in this study for the removal of Cr (VI) from synthetic wastewater. Andisols can be used as adsorbent because they are characterized by the presence of non-crystalline secondary minerals such as allophane and imogolite that show variable charge characteristics and have the ability to retain cations and anions. The adsorption of Cr on to two VAS from Mt. Isarog and Mandalagan (B-Horizon), Philippines, was carried out at ambient temperature using batch adsorption studies. The effects of different parameters such as amount of adsorbent, contact time, initial Cr concentration and pH of the solution were investigated. The results showed that the VAS from Isarog is more effective in the removal of Cr than in Mandalagan. The maximum removal efficiency of the Isarog soil for a Cr concentration of 10mg/L reached 89% with a dose of 20 g/L at a moderately acidic pH of 3. The Mandalagan soil on the other hand could remove only 65% at the same pH conditions and parameters. The difference in the removal of the two soils may be attributed to their physico-chemical properties in which the Isarog soil has higher clay content, porosity and lower bulk density. Isarog soil has fine particles with higher surface area and more active non-crystalline minerals and thus has higher removal efficiency than Mandalagan soil. Based on the results, the use of VAS from Isarog appears to be economical and an alternative to commercially available adsorbents for the removal of Cr from contaminated wastewater.