Effect of biogenic jarosite on the bio-immobilization of toxic elements from sulfide tailings.

The discharge of toxic elements from tailings soils in the aquatic environments occurs chiefly in the presence of indigenous bacteria. The biotic components may interact in the opposite direction, leading to the formation of a passivation layer, which can inhibit the solubility of the elements. In this work, the influence of jarosite on the bio-immobilization of toxic elements was studied by native bacteria. In batch experiments, the bio-immobilization of heavy metals by an inhibitory layer was examined in the different aquatic media using pure cultures of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. A variety of analyses also investigated the mechanisms of metals bio-immobilization. Among different tests, the highest metal solubility yielded 99% Mn, 91% Cr, 95% Fe, and 78% Cu using A. ferrooxidans in 9KFe medium after ten days. After 22 days, these percentages decreased down to 30% Mn and about 20% Cr, Fe, and Cu, likely due to metal immobilization by biogenic jarosite. The formation of jarosite was confirmed by an electron probe micro-analyzer (EPMA), X-ray diffraction (XRD), and scanning electron microscope (SEM). The mechanisms of metal bio-immobilization by biogenic jarosite from tailings soil confirmed three main steps: 1) the dissolution of metal sulfides in the presence of Acidithiobacillus bacteria; 2) the nucleation of jarosite on the surface of sulfide minerals; 3) the co-precipitation of dissolved elements with jarosite during the bio-immobilization process, demonstrated by a structural study for jarosite. Covering the surface of soils by the jarosite provided a stable compound in the acidic environment of mine-waste.

Minimization of metal sulphides bioleaching from mine wastes into the aquatic environment.

The continuous presence of toxic elements in the aquatic environments around mine tailings occurs due to bioleaching or chemical extraction promoted by the mining operations. Biogenic passivation treatment of tailings dams can be a new environment-friendly technique to inhibit the solubility of heavy metals. In spite of current bioleaching researches, we tried to minimize the mobility of the trace elements in the laboratory scale through the formation of a passivation layer in the presence of a mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. The X-ray diffraction (XRD) and scanning electron microscope (SEM) represented the jarosite generation as an inhibitory layer on the mineral surfaces of the tested materials. More detailed observations on electron probe micro-analyzer (EPMA) showed the co-precipitation of metals with the passivation layer. Thereby, the passivation layer demonstrates potential in elements immobilization which, in turn, can be optimized in the natural systems. Our working hypothesis was to exploit and optimize the formation of the passivation layer to maximize the immobilization of heavy metals (e.g., Cu, Cr). The optimization process of bioleaching experiments using indigenous bacteria caused a reduced solubility for Cu (from around 20% to 4.5%) and Cr (from around 30% to 10.6%) and the formation of 6.5 gr passivation layer. The analyses finally represented the high efficiency of the passivation technique to minimize metals bioleaching in comparison to earlier studies.

Fractionation and leaching of heavy metals in soils amended with a new biochar nanocomposite.

In this study, surface soils of the Bama Pb-Zn mine-impacted area were sampled for an area surrounding the mineral processing plant. After collecting 65 samples and analyzing them for initial Cu, Pb, Zn, and Cd metal contents, the area was zonated based on the concentration distribution using ordinary kriging in R. A single homogenous sample was prepared by mixing equal weights of each sample as being representative of the whole impacted area (ST). Next, a synthetic model soil (SM) was prepared according to the mean ST texture (SM), divided into two portions, where one portion was amended with a biochar composite (10% w/w) (SMA), both portions were artificially contaminated with Cu, Pb, Zn, and Cd (SMAC and SMC). The mixed soil ST, and the model soils SMC and SMAC, were subjected to soil sequential extraction procedure to determine the variations in fractionation of heavy metals. Results showed that the fractionation in the unamended model soil (SMC) was very close to the original real soil (ST). Moreover, in both amended and unamended soils, Cd and Pb had the highest and the lowest mobility, respectively. Zn and Cu showed intermediate mobilities. The performance of the amendment was evaluated using a 150-day column leaching test taking leachate samples at designated time intervals, and Cu, Pb, Zn, and Cd concentrations were analyzed. Results of column leaching were in good agreement with the soil fractionation as Cd and Pb showed the highest and the lowest mobilities, respectively. Leaching through the soil column was also simulated by HP1 model. Results of simulation found in acceptable proximity to the experimental data despite remarkable differences due to limitations in defining soil to the simulation system.

Simultaneous immobilization of heavy metals in soil environment by pulp and paper derived nanoporous biochars.

BACKGROUND: Biochars are the new generation of sustainable soil amendments which may be applied both to fertilize and remediate the impacted soils. The aim of current research has been synthesis and characterization of pulp and paper-derived biochars and determination of their mechanisms in simultaneous immobilization of heavy metals (Cu2+, Pb2+, and Zn2+) within contaminated soil. In a novel attempt, three different solid wastes of Mazandaran Wood and Paper Industries (barks and effluent sludge) were utilized to produce biochars. METHODS: The thermogravimetric behavior of the three selected biomasses were initially analyzed and the proper pyrolysis condition has been determined, accordingly. Alterations in surface active groups, before and after the pyrolysis process, have been detected by Fourier transform infrared (FTIR) spectroscopy. Elemental analysis and acid digestion procedure have been employed to measure C, H, N, S, O, and P contents of the biochars. Moreover, porosity and morphological characteristics have been monitored by Brauner-Emmet-Teller (BET) porosimetry and scanning electron microscopy (SEM). Batch adsorption tests have been designed and carried out. Finally, a set of soil sequential extraction experiments was performed over both amended/unamended soils which together with a post-sorption FTIR analysis, explained the possible competitive immobilization mechanism. RESULTS: Porosimetry study indicated the nanoporosity of the chars and the distribution pattern of adsorbed metals over the char samples. Batch sorption tests suggested remarkable uptake capacity for each char. The results of post sorption tests suggested that Cu is mainly involved in organic bonds of -NH2, -OH and -COOH groups, Pb forms insoluble hydroxide, phosphate or carbonate precipitates, and Zn is mostly engaged in the residual fraction. CONCLUSIONS: Accordingly, the bulky wastes are confirmed to have the potential to form sustainable biochar soil amendments.