Selenium Immobilization in Evaporation Pond Sediments by In Situ Precipitation of Ferric Oxyhydroxide

摘要

Selenium (Se) in agricultural drainage and evaporation pond waters in the semi-arid Central Valley of California can threaten water quality and is toxic to wildlife. This laboratory study focused on the development of an in situ chemical treatment to immobilize soluble Se in drained evaporation pond sediments by amendment with ferrous iron, which forms ferric oxyhydroxide (FeOOH). Three sediment samples from Kesterson Reservoir, Merced County, CA, with elevated levels of Se were characterized by conventional extraction techniques and then subjected to in situ Fe(ll) amendment. Total Se in the three untreated samples ranged from 4 to 29 mg kg~(-1), the majority of which was chemically reduced and insoluble. Soluble (0.25 M KCI-extractable) selenite [Se-(Ⅳ)] was > 90% immobilized in sediment when the added Fe(Ⅱ) was > 50 mmol of Fe(Ⅱ) kg~(-1) sed~(-1) whereas immobilization of soluble selenate [Se(Ⅵ)] was strongly dependent on the initial Se(Ⅵ) concentration. Extraction of Se(Ⅳ) by 0.01 M K_2HPO_4 decreased with increasing Fe(Ⅱ), indicating that Se(Ⅳ) immobilized by the FeOOH precipitation process was not accessible to the PO_4 extra etant. Exchangeable Se(Ⅵ) (0.01 M K_2HPO_4 extra etable) was partially immobilized in sediment depending on the initial Se(Ⅵ) concentration. Both Se(Ⅳ) and Se(Ⅵ) were occluded within FeOOH produced during Fe(Ⅱ) oxidation and hydrolysis. Iron(Ⅱ) salt amendment may be a viable in situ remediation technique for alkaline sediments containing elevated levels of trace elements such as Se.