Chengfang Yang

Sulfate migration in a river affected by acid mine drainage from the Dabaoshan mining area, South China

Sulfate, a major component of acid mine drainage (AMD), its migration in an AMD-affected river which located at the Dabaoshan mine area of South China was investigated to pursue the remediation strategy. The existing factors of relatively low pH values of 2.8-3.9, high concentrations of SO4(2-) (∼1940 mg L(-1)) and Fe(3+) (∼112 mg L(-1)) facilitated the precipitation of schwertmannite (Fe8O8(OH)6SO4·nH2O) in the upstream river. Geochemical model calculations implied the river waters were supersaturated, creating the potential for precipitation of iron oxyhydroxides. These minerals evolved from schwertmannite to goethite with the increasing pH from 2.8 to 5.8 along the river. The concentration of heavy metals in river waters was great reduced as a result of precipitation effects. The large size of the exchangeable sulfate pool suggested that the sediments had a strong capacity to bind SO4(2-). The XRD results indicated that schwertmannite was the predominant form of sulfate-bearing mineral phases, which was likely to act as a major sulfate sink by incorporating water-borne sulfate into its internal structure and adsorbing it onto its surface. The small size of reduced sulfur pools and strong oxidative status in the surface sediments further showed that SO4(2-) shifting from water to sediment in form of sulfate reduction was not activated. In short, precipitation of sulfate-rich iron oxyhydroxides and subsequent SO4(2-) adsorption on these minerals as well as water dilution contributed to the attenuation of SO4(2-) along the river waters.

Mineralogical characteristics of sediments and heavy metal mobilization along a river watershed affected by acid mine drainage

Trace-element concentrations in acid mine drainage (AMD) are primarily controlled by the mineralogy at the sediment-water interface. Results are presented for a combined geochemical and mineralogical survey of Dabaoshan Mine, South China. Developed sequential extraction experiments with the analysis of the main mineralogical phases by semi-quantitative XRD, differential X-ray diffraction (DXRD) and scanning electron microscopy (SEM) were conducted to identify the quantitative relationship between iron minerals and heavy metals. Results showed that schwertmannite, jarosite, goethite and ferrihydrite were the dominant Fe-oxyhydroxide minerals which were detected alternately in the surface sediment with the increasing pH from 2.50 to 6.93 along the Hengshi River. Decreasing contents of schwertmannite ranging from 35 wt % to 6.5 wt % were detected along the Hengshi River, which was corresponding to the decreasing metal contents. The easily reducible fractions exert higher affinity of metals while compared with reducible and relatively stable minerals. A qualitative analysis of heavy metals extracted from the sediments indicated that the retention ability varied: Pb > Mn > Zn > As ≈ Cu > Cr > Cd ≈ Ni. Results in this study are avail for understanding the fate and transport of heavy metals associated with iron minerals and establishing the remediation strategies of AMD systems.

Distribution and diversity of bacterial communities and sulphate‐reducing bacteria in a paddy soil irrigated with acid mine drainage

To investigate the effects of long‐term acid mine drainage (AMD) irrigation on the change in bacterial community and sulphate‐reducing bacteria (SRB) in a paddy soil.

Effects of eggshell addition on calcium-deficient acid soils contaminated with heavy metals

In this study, effects of water conditions (flooded, wet, or dry) and eggshell dosages (0, 0.1, 1.0, and 10.0 g/kg soil, respectively) on pH variation, content of unavailable state of heavy metals, form of heavy metals, and available nutritious element calcium (Ca) in acid soils contaminated with heavy metals were investigated, respectively. The soil samples were continuously cultivated indoors and analyzed by toxicity characteristic leaching procedure and community bureau of reference (BCR) sequential extraction procedure. The results showed that the addition of eggshell could effectively improve the pH of acid soil and increase it to neutral level. Moreover, the contents of unavailable state of heavy metals Cu, Zn, and Cd increased significantly. Furthermore, when the soil was cultivated under the flooded condition with 1.0 g/kg eggshell, the unavailable state of Cu, Zn, and Cd increased the most, and these heavy metals were transformed into residual state. On the other hand, the amount of available state of Ca increased to 432.19 from 73.34 mg/kg with the addition of 1.0 g/kg eggshell, which indicated that the addition of eggshell dramatically improved the available state of Ca. Therefore, eggshell could ameliorate the soil environment as it led to the decrease of available heavy metals and improvement of fertilization effectively. In a word, this study indicates that the addition of eggshell would be a new potential method for remediation of acid field soils contaminated with heavy metals.

Role of Dissolved Organic Matter in the Release of Chromium from Schwertmannite: Kinetics, Repartition, and Mechanisms

Dissolved organic matter (DOM) is an important factor influencing mineral biogeochemistry, although the role of labile DOM in the release of chromium (Cr) from schwertmannite, a mineral with high surface area, is unclear. In this study, the interaction of DOM with synthetic CrO-schwertmannite was investigated to better understand the potential fate of Cr in high-DOM environments. Minerals and their products were analyzed using Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Experiments were conducted at pH 3.2 or 6.5 for different lengths of time, with a shrinking core model developed to describe kinetic processes. The concentration of total Cr in solution reached a maximum when the pH was 6.5 and the concentration of L-tryptophan was 5 mM. The newly formed minerals were confirmed to be mixtures involving residual schwertmannite, goethite, ferrihydrite, and jarosite. A possible mechanism is proposed to be a ligand-controlled binary system, accompanied by possible reduction at acidic pH conditions (3.2), including mass transfer and charge transfer processes. This study gives a new perspective for understanding the reactivity and stability of schwertmannite in the environment; it also provides some predictions on the mobility and fate of Cr. These findings will help design remediation strategies for Cr contamination.

Fe- and S-metabolizing microbial communities dominate an AMD-contaminated river ecosystem and play important roles in Fe and S cycling

ABSTRACT Indigenous Fe- and S-metabolizing bacteria play important roles both in the formation and the natural attenuation of acid mine drainage (AMD). Due to its low pH and Fe-S-rich waters, a river located in the Dabaoshan Mine area provides an ideal opportunity to study indigenous Fe- and S-metabolizing microbial communities and their roles in biogeochemical Fe and S cycling. In this work, water and sediment samples were collected from the river for physicochemical, mineralogical, and microbiological analyses. Illumina MiSeq sequencing indicated higher species richness in the sediment than in the water. Sequencing also found that Fe- and S-metabolizing bacteria were the dominant microorganisms in the heavily and moderately contaminated areas. Fe- and S-metabolizing bacteria found in the water were aerobes or facultative anaerobes, including Acidithiobacillus, Acidiphilium, Thiomonas, Gallionella, and Leptospirillum. Fe- and S-metabolizing bacteria found in the sediment belong to microaerobes, facultative anaerobes, or obligatory anaerobes, including Acidithiobacillus, Sulfobacillus, Thiomonas, Gallionella, Geobacter, Geothrix, and Clostridium. Among the dominant genera in the sediment, Geobacter and Geothrix were rarely detected in AMD-contaminated natural environments. Canonical correspondence analysis indicated that pH, S, and Fe concentration gradients were the most important factors in structuring the river microbial community. Moreover, a scheme explaining the biogeochemical Fe and S cycling is advanced in light of the Fe and S species distribution and the identified Fe- and S-metabolizing bacteria.

Isotope geochemistry, hydrochemistry, and mineralogy of a river affected by acid mine drainage in a mining area, South China

The Hengshi River is a classic example of an acid mine drainage (AMD)-affected river located in the Dabaoshan mining area in southern China. This work utilized stable isotopes (δ34S and δ18O) and hydrochemical data of surface water samples as well as the mineralogical composition of sediment samples to evaluate the processes that affect the sulfate content in water of the Hengshi River. High concentrations of heavy metals (e.g. Fe: >347.78 mg L−1; Zn: >96.48 mg L−1) in the mud impoundment and relatively stable S isotope values (δ34S: −1.53‰ ∼ −0.88‰) in the upper stream suggested that most of the sulfates were derived from sulfide oxidation. Dilution and mineralization could decrease SO42− concentration, but had no significant influence on the isotope composition of SO42−. However, δ34S and δ18O increased with the decrease of SO42− concentration, accompanied by the elevation of pH and adequate organic matter being available, suggesting that bacterial (dissimilatory) sulfate reduction played an important role in the transformation of sulfate downstream. The methods used in this study can also be used in other natural systems. Furthermore, it is important to understand the causes of environmental pollution and to help environmental remediation.

Migration and fate of metallic elements in a waste mud impoundment and affected river downstream: A case study in Dabaoshan Mine, South China

Fate of metallic elements and their migration mechanisms in a waste mud impoundment and affected downstream were assessed. Physicochemical and mineralogical methods combined with PHREEQC calculation, statistical analysis and review of relevant literatures were employed. Results showed that the waste in mud impoundment had been severely weathered and acidized. Metallic elements exhibited high mobility and activity, with a mobility ranking order of Cd > Zn > Mn > Cu ≈ Cr > As ≈ Pb. Hydraulic transportation originating from elevation variation was the most important driving force for metallic elements migration. Although damming standstill was considered as an effective strategy for controlling coarse suspended particulate pollutants, metallic elements were still transported to the Hengshi River in both dissolved phase and fine suspended particle phase accompanied by the overflow of acid mine drainage. The concentrations of dissolved metallic elements were attenuated significantly along the Hengshi River within 41 km stretch. Precipitation/ co-precipitation of iron oxyhydroxides, especially schwertmannite, ferrihydrite and goethite minerals, were established as the most critical processes for metallic elements attenuation in river water. Accompanied by metals migration in the river, two pollution sensitive sites with notably high content of metals in the stretch of S6-S8 and S10, were identified in gently sloping river stretch.