| In recent years, emergent Cadmium(Cd) contamination issues in rivers in China happen frequently, which greatly threatens the basin ecosystem and the drinking water safety of residents downstream. Polyferric sulphate(PFS) is usually employed to deal with Cd contaminations in rivers due to its advantages of less dosage and good coagulation effect. After emergent control process, though concentration of Cd in surface water reaches the standard, a large amount of flocs containing Cd sank into the sediment, instead of being removed from the water system. However, the biological stability of Cd-loaded polyferric sulfate flocs in the sediment can rarely be brought to the forefront.Many studies have revealed that widely distributed dissimilatory iron-reducing bacteria(DIRB) in the sediment are able to reduce Fe(III) containing minerals, which brings about structural damage and re-release of heavy metals from minerals. Polyferric sulfate flocs are mainly consist of Fe(III) hydroxides, which poses a problem that if the Fe(III) in PFS flocs can be reduced by DIRB? If this is going to happen, how will the Fe and Cd in flocs transport and transform, and what factors will influence this process. Up to now, there is no related research in this area. To fill this gap, this present study found that a model DIRB strain, Shewanella oneidensis MR-1, could reduce the Fe(III) in Cd-loaded PFS flocs. Then, fate of Fe and Cd in flocs was systematically studied during the microbial reduction of Cd-loaded PFS flocs by S. oneidensis MR-1. On this basis, in order to simulate the effects to microbial Fe(III) reduction and Cd release in flocs caused by actual sedimentary circumstances, experimental groups with different conditions including disturbance, different initial Fe and Cd concentrations in flocs and addition of humus analog disodium anthraquinone-2,6-disulfonate(AQDS) were prepared.Results indicated that Cd-loaded PFS flocs were biolabile, and S. oneidensis MR-1 could successfully reduce the Fe(III) in Cd-loaded polyferric sulfate flocs and destroy the flocs. Then, biogenic Fe2+, Fe3+ and Cd2+ derived from broken flocs were relased into solution. As the reaction went on, newly formed secondary minerals including goethite(dominating phase) and magnetite came into being. After 48-h incubation, concentrations of Fe2+ and Cd2+ in solution presented a downward trend because plentiful hydroxyl adsorption sites(≡Fe-OH) on the surface of secondary minerals could re-anchor part of the released Cd2+ via outer sphere electrostatic adsorption and inner sphere complexation. In the meanwhile, carboxyl, alcoholic hydroxyl and other organic groups also contributed to the adsortption of Cd2+ in solution. Cd speciation analysis by sequential extraction illustrated that the majority of Cd(56.54% ~ 94.51% in the whole system) in solid phase existed in exchangeable and iron oxide surface bound forms, which would cause potential release risk to the environment.With respect to static condition(0 rpm), disturbance(120 rpm) was able to promote microbial Fe(III) reduction and Cd release from flocs. However, the equilibrium concentration of Cd2+ in solution in disturbance condition was lower than that in static condition; Increased initial Cd concentration in the system(Total Fe approx 300 mg L-1, Total Cd 0 mg L-1, 0.68 mg L-1, 1.86 mg L-1 and 4.18 mg L-1) brought constant decline of Fe(III) reduction rate, and the dominating phase in secondary minerals changed from magnetite to goethite. Meanwhile, maximum Cd2+ concentrations in solution were all above 400 μg L-1, but Cd release rate decreased due to increasing total Cd concentration; Increased initial Fe concentration in system(Total Cd approx 600 μg L-1, Total Fe 142.38 mg L-1, 284.29 mg L-1 and 512.86 mg L-1) contributed little to Fe(II) production, which led to decreased Fe(III) reduction rate. Cd release rate presented a rise and then followed by a decline; Addition of humus analog, AQDS, could also enchace Fe(III) reduction rate, thus promote Cd release from flocs. However, high concentration AQDS(1 mM) performed below expectations for the reason that it could be toxic to bacteria; Linear regression results of Fe(III) reduction rates and Cd release rates in the same condition or different conditions indicated that greater Fe(III) reduction rate could bring about more Cd release form flocs, thus cause greater harm to the environment.Based on the above, this present study will provide theoretical guidance for exploration of the biological stability of Cd-loaded PFS flocs resuled from emergent control of Cd contamination in rivers and the potential release risk of Cd in flocs, so as to prompt the development of new emergency disposal approaches to avoid secondary pollution of Cd brought by Cd-loaded flocs. |
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