Background. Soil contamination by hexavalent chromium is becoming a main environmental concern in China. This study developed a sewage sludge biochar modified by FeSO4 (CHBC) as a new reductant for Cr(VI)-contaminated soil. The effectiveness of CHBC-stabilized Cr(VI)-contaminated soil was investigated. Methods. Typical industrial Cr(VI)-contaminated soil in China was chosen as the medium. The total and Cr(VI) contents of the contaminated soil were 1014.6 and 973.5 mg/kg, respectively. The effectiveness of the Cr(VI)-contaminated soil stabilized by CHBC was investigated by the leaching test (US EPA method 1312), the simplified bioaccessibility extraction test (US EPA 2007 protocol and British Geological Survey), alkaline digestion (US EPA method 3060A), sequential extraction (BCR sequential extraction procedure), X-ray diffraction, and the risk assessment code test. Results. Results show that CHBC substantially reduced the leachability and Cr(VI) content of the contaminated soil. The leachability and content of Cr(VI) were lower than the thresholds of the Environmental Quality Standards of Soil in China for civil reuse and the China Environmental Quality Standards of surface water for civil use when the soil was stabilized with 10% dosage of CHBC. Conclusion. CHBC is highly efficient in stabilizing Cr(VI) and can effectively reduce the leachability and bioavailability of Cr in contaminated soil and thus feasible for stabilizing Cr(VI)-contaminated soil and shows potential for application in the field.

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The residual rich SiO2 slag was obtained for magnesium was leached from the boron mud by sulfuric acid. The SiO2 was leached from the residual slag by sodium hydroxide solution. The effect of the leaching temperature and the leaching time, the ratio of the sodium hydroxide solution to slag and the concentration of the sodium hydroxide solution on the leaching efficiency of SiO2 in the residual slag were also studied. The reasonable technical condition as follow : the leaching temperature is 130°C, the leaching time is 6 min, the concentration of the sodium hydroxide solution is 19mol·L-1, the ratio of the sodium hydroxide solution to slag is 2.5:1. The SiO2 leached rate can be stabilized about 85% under this condition. Silicon dioxide was produced by double carbonating the sodium silicate solution and its percentage purity is above 99%.

The composition and property of the boron mud is investigated by X-ray diffractometry (XRD), scanning electron microscope (SEM), thermogravimetry (TG), differential thermal analysis (DTA) and fourier transformed infrared spectroscopy (FITR). The mineral components of the boron mud are magnesite (MgCO3)、forsterite (Mg2SiO4)、hematite (Fe2O3)、dolomite (CaMg(CO3)2) and a small amount of magnesium sulfate and lizardite((Mg,Al)3[(Si,Fe)2O5](OH)4)). The decomposition reaction of the boron mud can be carried on at 400~600°C, and silicate of the boron mud can reaction with NaOH with the increase of temperature to about 1000°C.

The residual rich SiO2 slag was obtained for MgO and B2O3 was leached from the boromagnesite by ammonium sulfate. The SiO2 was leached from the residual slag by sodium hydroxide. The effect of the roasting time, calcination temperature and the ratio of sodium hydroxide to the residual slag on the leaching efficiency of SiO2 in the residual slag were also studied. The reasonable technical condition as follow : the roasting time is 60 min, the calcination temperature is 550°C, the ratio of the sodium hydroxide to the residual slag is 3.0:1. The SiO2 leached rate can be stabilized about 85% under this condition.The residual rich SiO