Study On Chromium (Ⅵ) Reduction By Organic Substances And Its Impact Factors

Chromium (Cr) is widely used in industries, such as steel production, leather tanning,corrosion prevention, dye, tannery industry and galvanic. As a result of unproper disposalof the wastes containing chromium, chromium danger to human, animal and plants hasbecome one of the most serious environmental contaminants. How to reduce the danger andmake chromium play the more important role has been the problem which the environmentprotectors care.Chromium mainly exsits in two forms in natural environments: Cr⁶⁺ and Cr³⁺. Thesetwo forms exhibit completely different mobility and toxicity. Cr⁶⁺ is high toxicity andsolubility. However, Cr³⁺ is much less toxic and has a strong affinity for soil particlesurfaces, displaying immobility. So, Cr⁶⁺ is often reduced to Cr³⁺ by chemical reductionagents to control chromium contaminatnts. In the paper, kinetics of the reduction of Cr⁶⁺ byorganic acids and its impact factors were studied in the homogenious and heterogeneoussystem.This thesis contains three chapters:In chapter I The Cr⁶⁺ reduction by cellulose and glucose was studied through batchexperiments at different temperatures and pH values. Then the effects of Mn²⁺ on thereaction were also studied.Results showed that reduction of Cr⁶⁺ by cellulose and glucosewas so slowly. Through the reaction, there is not any indication showing that Mn²⁺ had thecatalysis effects.In chapterâ…¡The reduction of Cr⁶⁺ by mandelic, malic and lactic acid was conductedat 25℃,35℃,45℃and a pH range of 4.0 to 6.0.The results showed that the ability in thereduction of Cr⁶⁺ by these organic acid was generally in the order: mandelic＞malic＞lactic. The influence of pH is the largeset to mandelic, but lactic is too easily effected bytemperature. We still conducted the catalysis of Mn²⁺ on the reduction of Cr⁶⁺ by the threekinds of acids at 25℃and pH=4.0. The results showed that Mn²⁺ catalyzed all the reaction,especially for mandelic acid. This proves that the different radicals make the differentability of the organic acid on the reduction of Cr⁶⁺. It indicates that the complex formation of Mn²⁺ with organic acid was the prerequisite.Complex of Mn²⁺with organic acidincreased the activity ofα-OH, resulting in the complex formation of Mn²⁺-α-OHorganic acid -Cr⁶⁺ and then accelerating Cr⁶⁺ reduction.In chapterâ…¢The catalysis of minerals on the reduction of Cr⁶⁺ by mandelic acid andmanlic acid was conducted at 25℃,35℃, 45℃and a pH range of 4.0 to 6.0. The resultsshowed that these minerals all had some catalysis effects on the reaction. The role ofminerals in accelerating the reduction of Cr⁶⁺ by mandelic acid and mamlic acids was in theorder: illite＞montmorillonitc＞＞kaolinte, which was agreement with the amounts ofexchangeable Mn²⁺ on the mineral surfaces. Exchangable Fe³⁺ on mineral surfaces alsoaccclerated the reduction of Cr⁶⁺ by citrate remarkably, but its catalytic role was less thanthat of Mn²⁺. Complex of Mn²⁺ with mandelic acid and manlic acid increased the activity ofa-OH of citrate, resulting in the complex formation of Mn²⁺-citrate-Cr⁶⁺ and thenaccelerating Cr⁶⁺ reduction. The study further pointed surface area of minerals was not thekey to catalyzing the reduction of Cr⁶⁺ by organic acids in soils.