(53c) Remediation of Cr (VI)-Contaminated Soil Using Hydrazine

The current status of technology for remediation of soil contaminated with Cr (VI) and our work on decontamination using hydrazine as the reducing agent will be described. Hexavalent chromium is a major metal-derived pollutant of water and soil which is toxic and established carcinogen. Mining and metallurgical industries activities as well as chrome chemicals and tanneries are major sources of release to the environment. Sites contaminated with Cr (VI) exist in countries such as USA, South Africa, Kazakhstan, New Zealand, UK, Canada, Italy and India.

Remediation of Cr (VI) essentially requires a suitable reducing agent or technique that converts it to much less toxic Cr(III). Both chemical reduction and bio-remediation have been practiced. The reducing agent should be non-toxic and any excess used should easily decompose. Use of calcium polysulfide, ferrous sulfate, sodium bisulfite, sodium dithionite, zero-valent iron and organic reducing agents such as molasses and glucose have been reported.

We have used hydrazine, a novel reducing agent, for remediation of Cr (VI) in soil. The reduction reaction is very fast and any excess can be easily taken care of. Remediation may be done in-situ by injecting the reagent at the site and ensuring its distribution in the soil mass. Alternatively, it may be done ex-situ either by mixing the soil for a sufficient time or by washing the Cr (VI) out of the soil and treating it with hydrazine in a stirred vessel or reactor. The last option is most advisable since the soil can be made free from both forms of chromium, Cr (VI) and Cr (III).

A literature report on the reaction of Cr (VI) with hydrazine is scanty. Kinetics of the reaction which is essential for comparison of its performance with competing reagents as well as for design of a reactor have not been reported so far. In this work we have done this study for remediation of soil washings. Soil slurries with different initial Cr (VI) concentrations were treated with hydrazine hydrate in a stirred reactor at different mole ratios. Samples were withdrawn from time to time, the suspended solids filtered out and the unreacted Cr (VI) estimated using the diphenylcarbazide technique. Reaction rates for different initial concentrations of Cr (VI), mole ratio of the reducing agents, pH and temperature were measured. The reaction was found to follow second order kinetics. The rate equation was developed by fitting the experimental data obtained for varying the initial concentrations of Cr(VI) and hydrazine at a given temperature and pH . The data fit was very good with a correlation coefficient of 0.984. The activation energy was determined from the Arrhenius plot. Further the second order rate constant was found to be linear in pH of the reaction medium.