Studies On Kinetics Of Chromium (Ⅵ) Reduction By Sulfude And Its Impact Factors

Chromium is widely used in dustry such as steel production, leather tanning, corrosion prevention, dye, tannery industries and galvanic. Aa a result of unproper disposal of the waste containg chromium, it is tested in many soils, nature waters, which leads to serious environmental pollution. Chromium mainly exsits in two forms in natural environment: chromium(â…¥) and chromium(â…¢).These two forms exhibit markedly different mobility and toxicity. Hexavalent chromium[Cr (â…¥)] is highly toxic, soluble, whereas trivalent chromium [Cr (â…¢)] is much less toxic and has a strong affinity for soil particle surfaces, displaying immobility. So, Cr(â…¥) is often reduced to Cr(â…¢) by reductors to control chromium contaminatnt. In the paper, we maily studied chromium (â…¥) reduction by sulfide in water and soil subsurfaces. The thesis includes three parts.In chapter â… , at constant temperature and in anaerobic conditions, the kinetics of chromium (Cr(â…¥)) reduction by sulfide and its affecting factors were conducted in batch experimental systems. The results showed that the reaction displayed in two stages when solution pH (between 7-9) kept stable and the initial concentration of sulfide was much higher than the chromium (Cr(â…¥) ) concentration. At the initial stage [Cr(â…¥) was consumed ranging from 50%~70%], the reaction followed the pseudo first-order and the plots of In c(Cr(â…¥)) versus reaction time (t) was linear. At the later stage [Cr(â…¥) remained about 0~60%], the plots of In c(Cr(â…¥)) versus t was deviated from the initial line and reaction rate was accelerated dramatically. The further study indicated that elemental sulfur was the main product of sulfide oxidation, which gradually formed colloid. Sulfide was adsorbed on the surfaces of colloid S, which accelerated the reaction rate remarkably. In addition, the reaction was first order with respect to c(H+).In chapter â…¡, at constant temperature and in anaerobic conditions, the kinetics of chromium(â…¥) reduction by sulfide was studied in three soils(red soils, yellow-brown soil, chernozem) in batch experimental systems. The results showed that Iron( â…¡) adsorbed by soil surfaces played an important role in accelerating chromium(â…¥) reduction by sulfide. The soil effect on the reaction rate was in correspondence with the contents of soil-extratable iron(â…¡). That was in the order of red soils