Sulfate Migration And Its Mechanism In A River Affected By Acid Mine Drainage In Metal Sulfide Mining Area

Sulfur is an essential nutrient element and has an average abundance of 0.05%(by weight) in the lithosphere. It can be highly concentrated in some environments, such as metal sulfides mining areas. Mining often speeds up the cycling of heavy metals and sulfur, and the acid mine drainage(AMD) is triggered by exposing metal sulfides to water and atmospheric oxygen. SO42- as a major component of AMD, its migration and effect in AMD affected river have not been studied sufficiently. In the present work, we surveyed the Hengshi River which was a sulfur-rich catchment affected by AMD in the Dabaoshan Mine, South China. To inform SO42- migration and its effects in the river, the following aspects were studied: physicochemical characteristics of the river water, sulfur distributions in water and sediment, heavy metals distributions and their relations with SO42-migration, mineralogy characteristics of sediment, SO42- redistributed in the water and sediment during phase transformation and dissolution of precipitates, and factors effected the formation and stability of sulfate-rich mineral schwertmannite in river. The main results were listed as follows:(1) Based on the field survey, the physical and chemical parameters showed the Hengshi River was polluted severely. The p H of AMD in the impoundment were 2.5~2.8, and the total dissolved solid(TDS) was up to 3157 mg/L. Affected by AMD, the p H of river water remained ~3.0 upstream of Liangqiao due to its strong buffering capacity. The water showed an oxidation state, with high level of DO and ORP. The TDS fell quickly along the river, indicated that the pollutants were diluted or transferred from water to sediment.(2) Free SO42- was the main specie of dissolved sulfur in river water, followed by Fe SO4+ and HSO4-, their distribution depended on p H value closely. SO42- concentration fell quickly in the river stretch where the p H was kept in 3 to 4, indicated forming some minerals which had strong capacity to bind SO42-. In the sediments, the Ex S was the dominant sulfur specie, but the reduced sulfur pools included ES, AVS and CRS accounted for a small fraction of the TS. The TS and Ex S in the river sediments showed distinct changes in different seasons, with the peak values in the site S6.(3) In the mud impoundment, the abundances rank of metals in the mud were Fe>Pb>As>Zn>Mn>Cu>Cr>Cd. Pb and As in the mud leached out greatly and then enriched on the surface mud. Zn and Mn also easily leached out but flowed away with water. Cu and Cr leached out slowly and slightly enriched on the surface mud. The concentrations of Fe, Mn and Zn in AMD were beyond the standards of natural surface water seriously, followed by Cd, Pb and Cu. In the river, SO42- migration affected the behaviors of heavy metals, the places where the heavy metals attenuated greatly were in tune with that happened for Fe3+ and SO42-. The great attenuation of heavy metals and SO42- in these locations may be attributed to the formation of iron second minerals. Factor analysis implied that the heavy metals in river sediments were concerned with three main components: pollutants source, formation of schwertmannite that related to Fe3+ SO42- and Cr O42-, and the adsorption capacity of sediments.(4) Affected by AMD, a layer of yellow or ochre precipitates settled on the river bed. The precipitates were characteristics of rich iron and sulphate, and composed of silicoaluminate, quartz and iron precipitates included schwertmannite and goethite. The iron precipitates presented an evolution with p H increasing along the Hengshi River. Surface sediments at S0 and S1 were mainly in forms of goethite and jarosite, but small amounts of schwertmannite were also observed at S1. Large amounts of schwertmannite presented at S2, S4 and S6 where the p H values were 2.8~3.9. Iron precipitates evolved into mixtures of goethite, ferrihydrite and schwertmannite at S7 and S8 where the p H values increased to 4.1~5.2, and only schwertmannite and goethite were detected at the river where p H was up to 5.2. XPS spectra showed that the sediments were composed of Fe, O, C, Al, Si, S and Pb, and formed the complex bonds of Fe3+-O2-, S6+-O2-, Si4+-O2-, Al3+-O2- and Pb2+-O2-. FTIR spectra showed SO42- migrated from water to sediment by the modes of monodentate inner-sphere complex and outer-sphere complex in sediments, the heavy metals was adsorbed on the sediments by exchanging with H+ on the surface hydroxyl groups. Acoording to the PHREEQC simulation of Fe3+activity and saturation index, the waters were supersaturated and had the potential to precipitate as goethite, jarosite, schwertmannite, ferrihydrite and hematite.(5) The minerals in sediment would suffer dissolution or phase transformation as the water conditions changed, SO42- in the minerals would redistributed between water and solid phase. AMD settling experiment in lab was used to simulate the change of hydraulic conditions. Compared with open and turbulent system in river water, the characteristics of AMD after settling in lab for 120 d and 330 d were different. The p H and the concentration of Fe, SO42-, Cu, Zn, Pb, Mn, As and Cd decreased. The precipitates obtained from AMD settling composed of goethite and schwertmannite, and had rich Fe of 46.20%~47.23% and S of 4.81%~5.01%. FTIR of these precipitates showed that SO42- were bound with Fe as [Fe-SO4] and [Fe-HSO4] through monodentate inner-sphere complex. Dialysis experiments were carried out to simulate the effects of dilution and p H variation for sediments. During the dialysis process, multitude H+ and SO42- released from the sediments, the complex specie of [Fe-HSO4] in sediments changed to [Fe-SO4]. The XRD of sediments had no marked change after 35 d dialysis period. Cu content in the sediments increased when the p H of dialysis water was higher to the value that the sediment original settled. Other heavy metals(e.g. Zn, Mn, Pb, Cd and As) lost significantly during dialysis experiment, illustrated that Zn, Mn, Pb, Cd and As would release from the sediment in rain season. The AMD polluted river had huge environmental risk.(6) Coexist ions and humic acid in river water could function with schwertmannite formation and its stability. XRD of synthesis minerals showed that adding 10~50 mg/L C of humic acid and 100~500 mg/L of Cr O42- in the mother liquids had no effect on the formation of schwertmannite. Humic acid could also improve the stability of schertmannite slightly, so as to SO42-. However, Cr O42- could compete with SO42- and result SO42- loss from mineral. In acid C2O42- solution, Fe3+ had strong complexation with C2O42-, resulting in the structure collapsing of schwertmannite. Above 90% SO42- would release from mineral in the conditions of 1:600(ratio of mineral to water) and 20 mmol/L acid C2O42-. In PO43-solution, SO42- in schwertmannite was replaced by PO43-, about 85% SO42- would release from mineral in the conditions of 1:600( ratio of mineral to water) and 5 mmol/L PO43-. As the differences of ionic radius and charge number between PO43- and SO42-, the replace of PO43-distorted the structure of schwertmannite and resulted in outer-sphere complex of PO43- in the mineral at last.In short, when AMD enter into the river, SO42- undergoed the processes of diffusion and dilution, precipitating with Fe3+ and heavy metal ions as iron hydroxide sulfate, adsorbing subsequently on the iron hydroxide sulfate, and releasing again as results of ion competition or sulfate-rich mineral dissolution or phase transformation. Microbial sulfate reduction was gradually activated downstream from the Shangba Village due to change of aqueous environmental factors.