(128e) Selenate Reduction from Mine Water By Redox Reaction and Adsorption Process

Authors: 
Banerjee, K., Veolia Water Solution & Tech
Das, T., Saint Martin's University
A treatment process was developed to reduce total selenium in mine water to â?¤5 µg/l. The treatment system utilizes a filtration and adsorption process. A proprietary iron-based media was used as an adsorbent. Using a sample from an abandoned coal mine site containing about 23µg/l of selenate, column studies were conducted for a period of six months. The objectives were to validate the applicability of this media in removing selenate from mine water. Several process parameters were investigated, including pH, ORP, adsorbent dosage, contact time, and the impact of co-occurring contaminants. Using an ICP-MS, selenium in water was analyzed. A high resolution X-ray Photon Spectroscopy (XPS), under nitrogen blanketing, was used to characterize the spent media. Results reveal that the adsorbent is capable of removing selenium to â?¤5 µg/l. The breakthrough curve showed that about 90,000 bed volumes of water were treated prior to reaching the target effluent selenium of â?¤ 5µg/l. The adsorption capacity of the media was not affected by the presence of co-occurring contaminants. Scaling or fouling was not observed within the column. No significant pressure drop (Î?P) was observed. The treated effluent contained about 2 mg/l of iron, and after aeration followed by direct filtration iron was reduced to < 0.1 mg/l.

Several kinetics models including pseudo first-order, pseudo second-order, power function, Elovich equation, and parabolic diffusion were used to analyze the data. The high r2 (square of the correlation coefficient) value for the Elovich model suggests that the data are generally best described by this equation for selenate adsorption at the experimental condition. The results suggest that the adsorption kinetics could potentially be a chemisorptions process. Literature data indicate that the Elevoch equation has general application to the chemisorption kinetics. Thermodynamics study results reveal that increasing temperature increased the kinetics and adsorption capacity. The calculated heat of adsorption value [Î?H] ranged between 130 and 150 KJ/mol. The X-Ray Photon Spectroscopy (XPS) results indicate that after adsorption, selenium onto the spent media existed in the form of elemental selenium [Se0]. It is apparent that the elemental selenium is the product of the redox reaction. It is anticipated that the selenate removal by the media could be a combination of redox reaction and chemisorptions process.

Selenium concentration in the Toxic Characteristics Leaching Procedure (TCLP) extract of the spent media was less than the regulatory limit of 1 mg/l, the spent media can be disposed as a non-hazardous material.