Mechanisms Of Detoxication In Semi-solid Fenton Process And Stabilization/Solidification For Organic Arsenical Industrial Solid Waste

Organic arsenical industrial solid waste (OAISW) from feed additive is a typical historical hazardous waste. The improper treatment, disposal or random stake can easily induce surrounding environment pollution. Focusing on the OAISW, this study investigated its harmless treatment processes and corresponding mechanisms by the combination of detoxication and stabilization/sodification evaluated by static, dynamic and ecological assessments, respectively.Firstly, the background value and nature of the OAISW were characterized. It indicated that the OAISW was co-polluted by organics and inorganics. Aniline (ArNH2) and o-nitroaniline (ONA), with the total concentrations of 90800±42 mg/kg and 1910±26 mg/kg respectively, was the top two kinds of organic pollutants. Moreover, the total leaching concentration of nitrobenzenes was 6500 mg/kg. Arsenic (As), with the total concentration and leaching concentration of 22353±11 mg/kg and 4600 mg/L, was the highest inorganic in the OAISW. Importantly, the As(III) with very high toxicity accounted for 45%-50%. The OAISW has high potential of pollution and needs urgent detoxication treatment.Subsequently, the detoxication treatment of the OAISW was carried. With respect to the generation of wastewater containing target pollutants once the traditional Fenton process was introduced, a new kind of semi-solid Fenton process (SSFP) designated for the detoxication of hazardous waste containing high level of organics was invented by adjusting the liquid-to-solid ratio (L/S). The effects, mechanisms and kinetics of the key factors of SSFP were systematically investigated. It shows that the SSFP has highest possibility on the OAISW detoxication, and the key factors was characterized as pH, Fe(II) and L/S. Based on the single factor and the Box-Behnken Design (BBD) tests, the pH, Fe(II), L/S and the target pollutant removal efficiency significantly fit a quadratic polynomial model. The optimum of pH, Fe(II) and L/S calculated by the model were 3.5,0.062 mol/kg and 1.77, respectively. Furthermore, the reaction rate (k) and half-life (t1/2) of SSFP were obtained by the kinetic model study. The general model about the relationship of target pollutant removal and mineralization efficiencies and pH, Fe(II) and L/S can well describe different kinds of SSFP scenarios. Correspondingly, the model between L/S and target pollutant concentrations can directly predict the optimal L/S. Finally, the multi-SSFP strategy for scenarios with different pollutant level can meet the environmental standard. Moreover, the total L/S is still lower than the traditional slurry system of≥5:1 and has greatly environmental effect.Based on the SSFP detoxication, the As in the OAISW was subject to further treatment by stabilization/solidification. Focusing on the combination effect and mechanism of stabilization reagents, solidification reagents, and environmental factors, the stabilization of As in the OAISW was evaluated by toxicity characteristic leaching procedure (TCLP), synthetic precipitation leaching procedure (SPLP) and sequential extraction procedure (SEP), respectively. It was observed that the As concentrations in TCLP and SPLP leachates were 1265 mg/L and 1057 mg/L, and the most noticeable fractions of F1 and F2 in SEP accounted for 28.4% after SSFP detoxication, respectively. The main mechanism of As stabilization in the OAISW was the single and combination adsorption and co-precipitation based on Fe(OH)3 colloid. The stabilization effect of single reagent, namely ferric sulfate (FS), magnesium chloride (MC) and calcium hydroxide (CH), showed that the lowest As concentrations in TCLP and SPLP leachates, and the lowest percentage of F1 and F2 can be observed in the FS treatment with the Fe:As of 2:1. However, the combination of FS+MC+CH treatment can promote the highest stabilization of As in the OAISW. As for the effects of Fe and Al, the highest stabilization of As in the OAISW was observed when the Fe:Al was 4:1 and the matrix pH of 6.0 adjusted by Ca(OH)2. The aging treatment can promote the further stabilization of As in the OAISW. However,3 days'aging treatment is enough for the evaluation of the stabilization effect. In the solidification treatment, the differences on the mechanism of hydration reaction between the 42.5# Portland cement (PC) and the Aluminate cement (AC) caused the difference on the As stabilization effect in the OAISW. The lower As concentrations in TCLP and SPLP leachates, the higher pressive strength, and the higher acid neutralization capacity were observed in the paste by AC solidification after 7 days' aging. AC has better solidification effect than 42.5# PC.Finally, the ecological toxicity of the OAISW after SSFP detoxication, and stabilization/solidification treatments were performed with the Chlorella pyrenoidosa as the indicator. It showed that the growth inhabitation of Chlorella pyrenoidosa, namely EC50 increasing, alleviated as the degree of harmless treatment. The chlorophyll a (chla) and chlorophyll b (chlb) increased and the ecological toxicity decreased correspondingly. The As concentrations distribution in the subcellular fraction of Chlorella pyrenoidosa showed that the different stabilization status of the OAISW has different target organ toxicity. For example, the As was mainly stored in the soluble fraction of the cell (P3) when the OAISW without any treatment. However, it was observed in the cellular organ and plasmalemma of the cell (P2) after SSFP detoxication. The As in the OAISW can significantly destroy the ultra microstructure of the Chlorella pyrenoidosa cell, namely the chloroplastid size significantly reduced and the starch grain greatly increased. It means that the OAISW has significantly ecological toxicity. However, those phenomena can be gradually recovered as the harmless treatment degree improved.In a word, the results of this study can provide some technical methods and parameters for the harmless treatment of the hazardous waste containing organics and inorganics. It also can supply some reference theory for the evaluation of hazardous waste harmless process.