Processes controlling metal ion attenuation in acid mine drainage streams

Two acid mine drainage streams have been investigated by detailed analysis of their sediments and waters, to obtain an understanding of the dominant processes which control the transport and attenuation of heavy metals under conditions of chronic high-level pollutant input. One of the water-courses has a thick hydrous iron oxide crust on its bed, where biotically mediated oxidation of ferrous iron resulted in precipitation of amorphous ferric hydroxide, along with substantial quantities of adsorbed silica, sulphate and Al and lesser quantities of As. Small amounts of K and Pb (and possibly hydronium) jarosites were also present in the sediments. Changes in pH and in the concentrations of Cu, Zn, and Cd appear to be mainly the result of dilution by seeps and tributaries.Although no sediment was recovered during collection of water samples from the second stream, saturation index calculations imply that precipitation should have been occurring. The observed down-stream loss of a number of elements supported this conclusion. The solids predicted to be precipitating were A1(OH)₃, Cu₂(OH)₂CO₃, and Fe(OH)₃. Observed decreases in the concentrations of Cd, Zn and Mn can be accounted for on the basis of dilution alone. However, the additional mechanism of neutralization by higher pH inflows is required to account for the decrease in hydrogen ion concentration downstream.The basis for a potentially useful new technique (congruent element analysis) which enables the identification of conservative components in streams is presented. Comparison of logarithmic concentration versus distance plots delineates the point where chemical removal mechanisms become important for each element.     

The passivation of calcite by acid mine water. Column experiments with ferric sulfate and ferric chloride solutions at pH 2

Column experiments, simulating the behavior of passive treatment systems for acid mine drainage, have been performed. Acid solutions (HCl or H₂SO₄, pH 2), with initial concentrations of Fe(III) ranging from 250 to 1500 mg L⁻¹, were injected into column reactors packed with calcite grains at a constant flow rate. The composition of the solutions was monitored during the experiments. At the end of the experiments (passivation of the columns), the composition and structure of the solids were measured. The dissolution of calcite in the columns caused an increase in pH and the release of Ca into the solution, leading to the precipitation of gypsum and Fe–oxyhydroxysulfates (Fe(III)–SO₄–H⁺ solutions) or Fe–oxyhydroxychlorides (Fe(III)–Cl–H⁺ solutions). The columns worked as an efficient barrier for some time, increasing the pH of the circulating solutions from 2 to 6–7 and removing its metal content. However, after some time (several weeks, depending on the conditions), the columns became chemically inert. The results showed that passivation time increased with decreasing anion and metal content of the solutions. Gypsum was the phase responsible for the passivation of calcite in the experiments with Fe(III)–SO₄–H⁺ solutions. Schwertmannite and goethite appeared as the Fe(III) secondary phases in those experiments. Akaganeite was the phase responsible for the passivation of the system in the experiments with Fe(III)–Cl–H⁺ solutions.     

A comparison of metal attenuation in mine residue and overburden material from an abandoned copper mine

The metal attenuation capacities of secondary acid mine water precipitates is dependent upon such factors as pH, ionic strength, the presence of competing ions, and tailings mineralogy. At the abandoned Spenceville Cu mine in Nevada County, California, approximately 6800 m³ of jarosite overburden and 28,000 m³ of hematite residue are potential sources of heavy metals loading to infiltrating surface waters. A column study was performed to assess the ability of the overburden and the residue to attenuate heavy metals from acidic mine drainage. The study information was needed as part of a remedial design for the abandoned mine, and was designed to simulate a worst-case scenario to examine the plausibility of backfilling a large open pit with the waste materials. Ten pore volumes of acidic mine drainage were allowed to pass through the materials, and the column effluents were analyzed for dissolved Fe, Al, Ca, Mg, Na, K, Mn, Cu, Zn, Pb and Ni using ICP-AES. The oxidation-reduction potential (Eh) was measured with a combination PtAg/AgCl electrode and also calculated from Fe(II) and Fe(III) measurements using the Nernst equation. Ion activities in solution and saturation index (SI) values for various solid phases were calculated using the geochemical speciation model MINTEQA2, and mineralogical compositions of fine (< 2 mm) and coarse ( > 2 mm) fractions were determined by XRD. Geochemical modeling of the column effluent compositions indicate that goethite, jarosite, jurbanite and gypsum are potential solid phases that may control metal solubilities in the column effluents. Excellent agreement was observed between the measured Eh values and those calculated from the activity ratio of Fe²⁺(aq) to Fe³⁺(aq). The large attenuation capacities for Cu and Zn exhibited by the jarosite overburden also suggest that solid solution substitution plays a large role in controlling metal concentrations in the pore waters. Relatively little metal attenuation, however, was provided by the hematite residue.     

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.     

Bioavailability of jarosite for stimulating acid mine drainage attenuation

Biological reduction of iron-sulfate minerals, such as jarosite, has the potential to contribute to the natural attenuation of acid mine drainage (AMD) sites. Previous studies of AMD attenuation at Davis Mine, an abandoned pyrite mine in Rowe Massachusetts, provided evidence of iron and sulfate reduction by indigenous bacteria. Jarosite is a large component of the sediment at Davis Mine and may play a role in AMD attenuation. In this study, microcosms were constructed with groundwater and sediment from Davis Mine and amended with glycerol, nitrogen and phosphorus (GNP) and naturally formed natrojarosite. Over time, higher total iron, sulfate, pH and sodium concentrations and lower oxidation–reduction potentials were observed in microcosms amended with GNP and jarosite, compared with unamended microcosms and killed controls. Geochemical modeling predicted jarosite precipitation under microcosm conditions, suggesting that abiotic processes were unlikely contributors to jarosite dissolution. SEM imaging at the jarosite surface showed microbial attachment. Microbial community composition analysis revealed a shift to higher populations of Clostridia, which are known to reduce both iron and sulfate. The results show that jarosite may be utilized as an electron acceptor by iron and/or sulfate reducing bacteria at Davis Mine and its presence may aid in the attenuation of AMD.     

Long-term interaction of wollastonite with acid mine water and effects on arsenic and metal removal

This paper reports the results of a laboratory experiment conducted to investigate the effects of wollastonite dissolution on removal of potentially toxic trace elements from stream waters affected by acid mine drainage (AMD). Nearly pure wollastonite was treated with natural acid mine water (pH 2.1) for different lengths of time (15, 30, 50 and 80 days). The compositional and textural characterization of the solid reaction products suggests that wollastonite was incongruently dissolved leaving a residual amorphous silica-rich phase that preserved the prismatic morphology of the parent wollastonite. The release of Ca into solution resulted in a pH increase from 2.1 to 3.5, and subsequent precipitation of gypsum as well as poorly crystallized Fe–Al oxy-hydroxides and oxy-hydroxysulfates whose components derived from the AMD solution. A geochemical modeling approach of the wollastonite–AMD interaction using the PHREEQC code indicated supersaturation with respect to schwertmannite (saturation index = 10.7–15.7), jarosite (SI = 8.7–10.2), alunite (SI = 5.1), goethite (SI = 4.7) and jurbanite (SI = 2.2). These secondary phases developed a thin coating on the reacted wollastonite surface, readily cracked and flaked off upon drying, that acted as a sink for trace elements, especially As, Cu and Zn, as indicated by their enrichment relative to the starting wollastonite. At such low pH values, adsorption of As oxyanions on the positively charged solid particles and coprecipitation of metals (mainly Cu and Zn) with the newly formed Fe oxy-hydroxides and oxy-hydroxysulfates seem to be the dominant processes controlling the removal of trace elements.     

Mineralogical attenuation for metallic remediation in a passive system for mine water treatment

Passive systems with constructed wetlands have been consistently used to treat mine water from abandoned mines. Long-term and cost-effective remediation is a crucial expectation for these water treatment facilities. To achieve that, a complex chain of physical, chemical, biological, and mineralogical mechanisms for pollutants removal must be designed to simulate natural attenuation processes. This paper aims to present geochemical and mineralogical data obtained in a recently constructed passive system (from an abandoned mine, Jales, Northern Portugal). It shows the role of different solid materials in the retention of metals and arsenic, observed during the start-up period of the treatment plant. The mineralogical study focused on two types of materials: (1) the ochre-precipitates, formed as waste products from the neutralization process, and (2) the fine-grained minerals contained in the soil of the wetlands. The ochre-precipitates demonstrated to be poorly ordered iron-rich material, which gave rise to hematite upon artificial heating. The heating experiments also provided mineralogical evidence for the presence of an associated amorphous arsenic-rich compound. Chemical analysis on the freshly ochre-precipitates revealed high concentrations of arsenic (51,867 ppm) and metals, such as zinc (1,213 ppm) and manganese (821 ppm), indicating strong enrichment factors relative to the water from which they precipitate. Mineralogical data obtained in the soil of the wetlands indicate that chlorite, illite, chlorite–vermiculite and mica–vermiculite mixed-layers, vermiculite, kaolinite and goethite are concentrated in the fine-grained fractions (<20 and <2 μm). The chemical analyses show that high levels of arsenic (up to 3%) and metals are also retained in these fractions, which may be enhanced by the low degree of order of the clay minerals as suggested by an XRD study. The obtained results suggest that, although the treatment plant has been receiving water only since 2006, future performance will be strongly dependent on these identified mineralogical pollutant hosts.     

High efficiency of heavy metal removal in mine water by limestone

The removal of Cd, Cu, Ni and Zn from dilute mine water by using several geological materials including pure limestone, sand, carbonaceous limestone and brecciated limestone was performed on a laboratory scale. The results showed that to add geological materials in combination with sodium carbonate injection would notably enhance the efficiency of heavy metal removal to varying degrees. Pure limestone was found the best one among the four materials mentioned above for removing heavy metals from mine water. The removal efficiencies of pure limestone when it is ground as fine as 30–60 meshes are 58.6% for Cd, 100% for Cu, 47.8% for Ni, and 36.8% for Zn at 20°C. The optimum pH is about 8.9 to 9.1. The mechanism of higher effective removal, perhaps, is primarily due to co-precipitation under the control of calcite-related pH value. According to this research, Na₂CO₃ injection manners, including slug dosing and drip-wise, seemed to have little impact on the efficiency of heavy metal removal.     

Refining the estimation of metal loads dissolved in acid mine drainage by continuous monitoring of specific conductivity and water level

The accurate estimation of metal loads transported by streams is necessary to calculate reliable mass transfers of metals between compartments, both at local and global scales. This estimation is particularly relevant in the case of the Tinto and Odiel Rivers (SW Spain) due to their significant contribution to the total metal transfer from continents to the ocean. At a local scale, the metal load transported by streams plays a key role in predicting the biogeochemical evolution of water reservoirs affected by Acid Mine Drainage (AMD). This work uses the relationships between specific conductivity (SC) and dissolved elements to calculate the metal load of the River Meca, a tributary of the Odiel. The SC and the water level were continuously monitored from April 2009 to June 2010. Water samples were also collected and measurements of the discharge were carried out manually once a month. The relationships between the SC and the concentration of dissolved elements are, in general, very good (R² > 0.90). However, some key elements such as Fe show a very poor correlation. A simple methodology based on the MIX code (a maximum likelihood method to estimate mixing ratios) was used to elucidate their different behaviours. During the dry period (April–December, 2009) the Fe concentration was lower than that deduced from the SC recorded value due to the precipitation of Fe-oxihydroxides, which also reduced the concentrations of As, Cr, Pb and, to a lesser extent, Cu. At the same time Na, Sr, Ca and Li were enriched because of the higher interaction with the riverbed materials. Correlations between the SC and the metal concentration improved significantly when each period was considered separately. A second dry period (April–June 2010) shows high SC values, although no dissolution/precipitation of solid phases is evidenced. This indicates that SC alone is not enough to predict the dissolved metal loads in Mediterranean AMD streams. The metal load transported by the River Meca was determined for the hydrological year 2009/10 as 1933 ± 129 tonnes of Fe, 990 ± 155 of Al and 378 ± 41 of Zn.     

Arsenic immobilization in water and soil using acid mine drainage sludge

Adsorption onto Fe-containing minerals is a well-known remediation method for As-contaminated water and soil. In this study, the use of acid mine drainage sludge (AMDS) to adsorb As was investigated. AMDS is composed of amorphous particles and so has a large surface area (251.2 m² g⁻¹). Here, adsorption of both arsenite and arsenate was found to be almost 100%, under various initial AMDS dosages, with the arsenate adsorption rate being faster. The optimum pH for As adsorption onto AMDS was pH 7.0 and the maximum adsorption capacities for arsenite and arsenate were 58.5 mg g⁻¹ and 19.7 mg g⁻¹ AMDS, respectively. In addition, experiments revealed that AMDS dosages decreased As release from contaminated soil. Therefore, the AMDS used in this study was confirmed to be a suitable candidate for immobilizing both arsenite and arsenate in contaminated soils.     

Influence of pH on mineral speciation in a bioreactor simulating acid mine drainage

Mineral precipitates formed under conditions representative of acid mine drainage were prepared by oxidizing 0.1 M FeS0₄ · 7H₂0 solutions at 24°C and pH 2.3, 2.6, 3.0, 3.3 and 3.6 using a bioreactor and a strain ofThiobacillus ferrooxidans. The oxidation of dissolved Fe²⁺ was monitored colorimetrically and was completed within 90 to 120 h at all pHs. Schwertmannite, Fe₈O₈(OH)₆SO₄, was a major component of the precipitates and was the only phase formed at pH 3.0. Jarosite, (H,Na,K)Fe₃(OH)₆(SO₄)₂, increased in abundance with decreasing pH whereas goethite, α-FeOOH, appeared at pH 3.3 and 3.6. A similar relationship between pH and mineralogy has been reported in natural specimens of mine drainage ochres.     

Field assessment of acid mine drainage contamination in surface and ground water

Both sulfate and conductivity are useful indicators of acid mine drainage (AMD) contamination. Unlike pH, they are both extremely sensitive to AMD even where large dilutions have occurred. The advantage of using sulfate to trace AMD is that unlike other ions it is not removed to any great extent by sorption or precipitation processes, being unaffected by fluctuations in pH. These two parameters are also closely associated as would be expected, as conductivity is especially sensitive to sulfate ions. Therefore, as sulfate analysis is difficult in the field, conductivity can be used to predict sulfate concentration in both AMD and contaminated surface waters using regression analysis. Most accurate predictions are achieved by using equations given for specific conductivity ranges or AMD sources. There is also potential to use conductivity to predict approximate concentrations of key metals when the pH of the water is within their respective solubility ranges.     

Competitive effect of organic anions on phosphorus attenuation capacity of acid mine drainage floc

This study examines the phosphorus attenuation capacity of acid mine drainage (AMD) floc (also called ocher or sludge)—generated by neutralizing AMD with ammonia, lime, and sodium hydroxide—in the face of competition from two major organic companions, citrate and oxalate, of phosphorus in manure. Lime-treated floc (LF) was the least effective of the three flocs in attenuating both inorganic phosphorus and organic phosphorus (OP: represented by inositol hexaphosphate or phytate). Out of the remaining two flocs, ammonia-treated floc (AF) attenuated more inorganic phosphorus and less organic phosphorus than did sodium hydroxide-treated floc (SF). Increasing citrate:phosphorus molar ratio in the solution from 0:1 to 1:1 decreased the inorganic P attenuation capacity (IPAC) from 53 to 29 % in AF, 33 to 16 % in LF, and from 49 % to 27 % in SF at pH 4. The corresponding figures for organic P attenuation capacity (OPAC) were from 73 to 54 % in AF, 58 to 45 % in LF, and from 76 to 58 % in SF. Increasing oxalate:phosphorus molar ratio from 0:1 to 1:1 decreased IPAC and OPAC similarly, but to a lesser extent. The competitive influence of citrate and oxalate went on weakening with increase in pH. A likely increase in pH following prolonged manure application may undermine the competitive ability of citrate and oxalate. The study shows that manure P attenuation potential of waste of AMD treatment, notwithstanding the peer anion competition to P, warrants its effectiveness in controlling buildup of P in heavily manured soils.     

Effects of iron on arsenic speciation and redox chemistry in acid mine water

Concern about arsenic is increasing throughout the world, including areas of the United States. Elevated levels of arsenic above current drinking-water regulations in ground and surface water can be the result of purely natural phenomena, but often are due to anthropogenic activities, such as mining and agriculture. The current study correlates arsenic speciation in acid mine drainage and mining-influenced water with the important water-chemistry properties Eh, pH, and iron(III) concentration. The results show that arsenic speciation is generally in equilibrium with iron chemistry in low pH AMD, which is often not the case in other natural-water matrices. High pH mine waters and groundwater do not always hold to the redox predictions as well as low pH AMD samples. The oxidation and precipitation of oxyhydroxides deplete iron from some systems, and also affect arsenite and arsenate concentrations through sorption processes.     

Heavy metal geochemistry of the acid mine drainage discharged from the Hejiacun uranium mine in central Hunan,China

The acid mine drainage (AMD) discharged from the Hejiacun uranium mine in central Hunan (China) was sampled and analyzed using ICP-MS techniques. The analyzing results show that the AMD is characterized by the major ions Fe_Total, Mn, Al and Si, and is concentrated with heavy metals and metalloids including Cd, Co, Ni, Zn, U, Cu, Pb, Tl, V, Cr, Se, As and Sb. During the AMD flowing downstream, the dissolved heavy metals were removed from the AMD waters through adsorption onto and co-precipitation with metal-oxhydroxides coated on the streambed. Among these metals, Cd, Co, Ni, Zn, U, Cu, Pb and Tl are negatively correlated to pH values, and positively correlated to major ions Fe, Al, Si, Mn, Mg, Ca and K. The metals/metalloids V, Cr, Se, As and Sb are conservative in the AMD solution, and negatively-correlated to major ions Na, Ca and Mg. Due to the above different behaviors of these chemical elements, the pH-negatively related metals (PM) and the conservative metals (CM) are identified; the PM metals include Cd, Co, Ni, Zn, U, Cu, Pb and Tl, and the CM metals V, Cr, Se, As and Sb. Based on understanding the geochemistry of PM and CM metals in the AMD waters, a new equation: EXT = (Acidity + PM)/pH + CM × pH, is proposed to estimate and evaluate extent of heavy-metal pollution (EXT) of AMD. The evaluation results show that the AMD and surface waters of the mine area have high EXT values, and they could be the potential source of heavy-metal contamination of the surrounding environment. Therefore, it is suggested that both the AMD and surface waters should be treated before they are drained out of the mine district, for which the traditional dilution and neutralization methods can be applied to remove the PM metals from the AMD waters, and new techniques through reducing the pH value of the downstream AMD waters should be developed for removal of the CM metals.     

山底河流域煤矿酸性矿井水野外监测

酸性矿井水在我国鲁西南、山西、内蒙、云南和贵州等煤矿区普遍存在,酸性矿井水其pH往往在2~5之间,高SO₄²⁻、HB、TDS、Fe、Mn。这些物质进入地下水、地表水或土壤后,会对其造成严重危害。文章选择山西阳泉市典型废弃煤矿区山底河流域为研究区,通过水文地质调查,水文地质钻探,水文地质剖面等方法阐述山底流域地层岩性,水文地质条件概况,得出受煤矿开采影响,与天然条件下相比山底河流域的地表水和地下水的补给、径流、排泄条件均发生了根本变化。补给通过破坏产生的导水裂隙带运移,以垂向运动为主;径流通过坑道,导水裂隙带运移,以横向运动为主;排泄以矿坑排水和泉水溢出方式为主。并简述山底河流域煤矿酸性矿井水试验站观测站分布情况与水化学特征。     

用卫星高光谱数据提取江西德兴铜矿矿山废水的pH值污染指标

在对德兴铜矿矿山废水的光谱特征深入分析研究的基础上,总结了不同类型水体(酸性水、碱性水以及河流水)的特征光谱,并利用地物谱特征开展矿山废水pH值污染指标提取研究。针对水体光谱反射率低、特征光谱不明显的特点,采用矿区卫星Hyperion高光谱数据,应用ISA算法和掩膜技术识别出水体分布并进一步与MNF变换有效结合,根据波段散点图进行不同pH值水体的有效分割。为矿山废水污染的诊断和监测提供了新技术和理论支撑。