| Iron oxides including goethite, lepidocrocite, hematite, and maghemite were synthesized in the laboratory. Soil colloids (<2μm clay fraction of the soil) was separated by sedimentation from typical soil types including latosol, latosolic red soil, and red soils developed from different parent materials including granite, basalt, Quaternary Period red soil, and sandstone shale, etc. selected in southern China. Total Fe, Free iron oxides, amorphous, and complexation Fe content were analyzed by normal chemical method. XRD, FTIR, BET, SEM-EDX were used to determine the crystal structure, surface area, surface micro- appearance and constituents. Transformation of pentachlorophenol, the target pollutant of this investigation on the surface of the iron oxides and colloids was investigated by batch kinetic method under anoxic conditions at 25±1 oC. Effects of iron oxides dosage, types, pH of the suspension, Fe(II) concentration, polycarboxylic acids, Mn(II) and other divalent transition metal ions including Co(II), Ni(II), Cu(II), Zn(II) on the transformation were also investigated by batch kinetic method mentioned before.Results obtained from XRD and SEM etc. showed that the common iron oxides were synthesized successfully, each of them with a relative single crystal. PCP can be transformed slowly in the suspension contain iron oxides only, the removal percentage was 8.49%, 71.2%, 36.0%, and 2.71%; the pseudo-first order rate constant of the transformation was 0.014±0.001, 0.144±0.008, 0.062±0.003, and 0.004±0.001 d-1 in the presence of goethite, lepidocrocite, hematite, and maghemite only, respectively. On basis of goethite, PCP transformation depended strongly on system pH with an optimal value at 4.0.The results also showed that abiotic transformation of PCP could be enhance significantly by additional polycarboxylic acids including oxalic acid, citric acid, succinic acid, and tartaric acid, and also additional Fe(II) or Mn(II) in the suspension of iron oxides including goethite, lepidocrocite, hematite, and maghemite. Addition of Cu(II) could also enhance the transformation of pentachlorophenol, only when Fe(II) exists in the suspension contain iron oxides. The pseudo-first order rate constant of reductive transformation pentachlorophenol increased with the increasing Fe(II) amount added, and PCP transformation also depended strongly on the oxalate concentration with an optimal value at 2.0 mM. Polycarboxylic acids including oxalic acid, citric acid, succinic acid, and tartaric acid, and also additional Fe(II) could enhance PCP reductive dechlorination greatly and also promote the generation of chloride ion in the suspension of goethite. PCP transformation depended strongly on surface-bound Fe(II) species content in the presence of additional Fe(II) only, or polycarboxylic acids with a certain concentration, and on the mole ratio of oxalate and surface-bound Fe(II) species (oxalate/Fe(II)) in the presence of oxalic acid with different concentrations.The results showed that PCP can be transformed at the interface of soil colloid. The average percentage of PCP removal was (35.09±17.80)% within nineteen days in the presence of soil colloids. And there is much difference exists among different soil colloids. On the part of soil type, the average percentage of PCP removal in the suspension contain soil colloids followed the order as latosol > paddy soil > latosolic red soil > red soil. On the part of parent material, the average percentage of PCP removal in the suspension contain soil colloids followed the order as basalt > sandstone shale > Quaternary Period red soil. The average rate constant of the transformation was (0.023±0.010) d-1 in the presence of soil colloids. And there is also much difference exists among different soil colloids. On the part of soil type, the rate constant of PCP transformation in the suspension contain soil colloids followed the order as latosol > paddy soil > latosolic red soil > red soil. On the part of parent material, the rate constant of PCP transformation in the suspension contain soil colloids followed the order as basalt > sandstone shale > Quaternary Period red soil.Addition of 1.0 mM Fe(II) or oxalic acid could enhance the transformation of pentachlorophenol in the suspension contain 10 g L-1 soil colloids. Take no account of soil type and parent material, the average percentage of PCP removal and the rate constant of PCP transformation increased 67.58±17.65% and 1.85±0.91 times by the addition of 1.0 mM Fe(II), and 1.31±0.51 and 2.65±0.79 times by the addition of 1.0 mM oxalic acid, respectively.Enhancement of Fe(II) and oxalic acid on the transformation of pentachlorophenol in the presence of soil colloids might be contributed to the formation of surface-bound Fe(II) on the surface of soil colloids. Soil pH, organic mater content, surface constituent of soil colloid might also play an important role in the transformation of pentachlorophenol on surface of soil colloids.Aimed at contamination of organic chloride pesticide on the red soil resource, taken the hot spot of international research on interface chemistry of soil colloids/water into account, reductive transformation of pentachlorophenol, the target pollutant of this investigation on the surface of the iron oxides and colloids was investigated by batch kinetic method under anoxic conditions at 25±1 oC. Effect of parent material and soil type, physiochemical characters including total Fe content, free Fe content, amorphous Fe content on the transformation were also investigated to disclose the relationship between reductive dechlorination reactivity of soil colloids and soil formation process. This work indicated that decontamination of reducible organic pollutants in natural environments could be effectively enhanced by the increase of surface-bond Fe(II) species content in the system, and will be helpful to understand the natural attenuation of reducible organic pollutants in the contaminated sediments, subsurface environments, and soil profiles, and would also provide basic understanding to predict the fate of PCP in natural environments. This investigation provides a feasible remediation technology and might potentially be used in conjunction with the existing remediation strategies. |
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