| Arsenic and chromium are common contaminant of water environmental pollution. Arsenic is highly toxic substance, it is listed as the first kind of carcinogenic substance by the international agency for research on cancer(IARC). Chromium toxicity is mainly caused by Cr(Ⅵ), its toxicity is about 100 times of Cr(Ⅲ), and has strong "three induced" effect to creature.At present, not only the study of containing arsenic and chromium wastewater treatment is not a lot, but also the technical is imperfections. The method of the ferrous- lime precipitation was used widely, but the stability of the sediment is poorer, so the arsenic, chromium and other heavy metals are easily return to the nature by the form of ion once again, and causing secondary pollution. Therefore, this research have improved the traditional ferrous- lime precipitation process, and optimize the various factors to make sediment achive stablest state during meeting the treatment effect. Research target is mainly as follows: investigate pH, ferrous dosing amount, reaction time, temperature, aeration oxidation time, Ca/As mole ratios effect to the removal of the Cr( Ⅵ) and total chromium, total arsenic; Study and exploration of the arsenic chromium removal mechanism and dynamic model; optimize the treatment conditions to make the stability of the sediment to the best and arsenic and chromium leaching concentration reaches Integrated wastewater discharge standard(GB 8978—1996); Considering the removal of arsenic, chromium conditions and the stability of sediment to determine the arsenic, chromium wastewater treatment optimum parameters, and characterize the sediment.The results show that, while reaction pH < 2 or>6, Fe(Ⅱ)/Cr(Ⅵ) mole ratios>4.5 in 25℃, Cr(Ⅵ) can be restored at a rapid rate, and the removal rate of Cr(Ⅵ) can reach more than 99% after reaction 40 min. Between initial reaction pH, Fe( Ⅱ)/Cr(Ⅵ) mole ratios, temperature and the reduction of Cr( Ⅵ) have a quadratic polynomial( y = 0.0025x2-0.0193x+0.0472, R2 = 0.9236), index(y = 0.0015e0.2701 x,R2=0.989)and linearity relation(y = 3473.3x-7.6322,R2 = 0.9677)respectively.When the pH > 2, the arsenic(0.003mol/L) can promote Cr(VI) by Fe( Ⅱ) reduction, and suppress Cr(III) removed. The presence of chromium ion(0.008 mol/L) has no effect on the removal of arsenic, and calcium ion(0.015 mol/L) has no effect on chromium removal also, but has a promoting effect on arsenic removal.Considering the effect of containing arsenic, chromium wastewater treatment and stability of the resulting sediment, obtain new technology optimum treatment conditions: the eaction temperature is 25, the Fe(Ⅱ)/Cr(Ⅵ) mole ratio is 8, aeration time is 50 min, Ca/As molar ratio is 5, pH value is 6. The removal effect of arsenic, chromium respectively reached 98.8%, 99.3% and 99.8%, and arsenic, chromium leaching concentration to achieve 1.78 mg/L and 2.59 mg/L, far below the limited concentration of Identification Standards for Hazardous Wastes: Leaching Toxicity(GB/T 5085.3-2007), and reached the safe stacking requirements.To SEM scanning of sediment what produced from optimal conditions, it was found that the particle size of the optimized sediment is bigger than optimized front, and large sediment particle size contribute to improve the stability of sediment, reducing the leaching concentration of arsenic and chromium.Effect of temperature on sediment experiments showed that arsenic and chromium leaching concentration is greatly influenced by environment temperature, higher temperature could increase the arsenic, chromium leaching concentration. Therefore, sediment should be stacked at a lower temperature environment. |
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