Selenium Removal from Agricultural Drainage Water: Lab Scale Studies

This report is an evaluation of the selenium removal from agricultural drainage water and synthetic solutions contaminated with high amounts of selenium. Batch and kinetic studies were conducted on the removal of selenium and the effectiveness of various remediation materials was determined. The agricultural drainage water samples were obtained from San Joaquin Valley and provided by Department of Water Resources, California. Nanosized zerovalent NiFe and Fe particles rapidly reduced and immobilized selenate from aqueous solutions. Nearly 100% selenate removal was obtained in five hours under most conditions. The data show that, at identical solids loading, the use of NiFe particles as compared to Fe and Ni particles accomplished greater than 42% and 56% removal, respectively. Reduction of selenium using bimetallic nanosized NiFe particles resulted in nearly complete selenium removal from agricultural drainage water samples. The presence of sulfates in the aqueous solutions decreased the degree of removal. However, sufficient removal is possible using these particles and can be used to achieve the 10 ppb USEPA mandated levels. Immobilization of selenate with barium chloride also appears to be an effective method with the final cleanup of selenium with NiFe bimetallic particles. The adsorption studies on both selenite and selenate removal showed that the commercially available sorbents such as γ alumina, α alumina and activated carbon showed some promising results for selenite removal. However, they were found to be completely ineffective for selenate removal, which is one of the predominant selenium species in the agricultural drainage water. The data also showed that γ alumina provided higher selenite removal percentages (99%) as compared to α alumina (94%), activated carbon (87%) and chitin (49%). The selenite removal was found to decrease with increasing initial Se (IV) concentration in the solution. Adsorption capacities of the adsorbents are reported in terms of their Langmuir adsorption isotherms. The adsorption capacity (on unit mass basis) of the adsorbents for selenite is in the order: chitin < activated carbon < α alumina < γ alumina. Generally, low pH of the solution resulted in favorable selenium removal. Adsorption experiments at controlled pH conditions confirmed that surface charge density can have significant influence in equilibrium uptakes of these oxyanions. Modification of the carbon surface by copper cations significantly enhanced the equilibrium uptakes of both selenite and selenate. The surface modification of activated carbon resulted in up to 68% and 217% enhancement in uptakes from aqueous solutions containing 1 mg/L selenite and selenate, respectively. Similarly, the increase in selenite and selenate uptakes with the use of modified Southern Illinois University fly ash derived char carbon (SIUF_C) was evaluated to be 240 and 80%, respectively, while those employing modified Lake of Egypt fly ash derived char carbon (LOEF_C) showed an increase of 155 and 128%, respectively, over the as-extracted carbons. Sulfate and nitrate were observed to hinder the uptake selenite, while chloride did not affect selenite uptake. This report was submitted by the Southern Illinois University, Carbondale (SIU) in fulfillment of Contract Agreement Number: 4600001985 under the sponsorship of the Department of Water Resources, California (DWR). This report covers the experimental work done from July 2001 through June 2004.