title: Groundwater arsenic removal by coagulation using ferric(III) sulfate and polyferric sulfate: A comparative and mechanistic study.
abstract: Elevated arsenic (As) in groundwater poses a great threat to human health. Coagulation using mono- and poly-Fe salts is becoming one of the most cost-effective processes for groundwater As removal. However, a limitation comes from insufficient understanding of the As removal mechanism from groundwater matrices in the coagulation process, which is critical for groundwater treatment and residual solid disposal. Here, we overcame this hurdle by utilizing microscopic techniques to explore molecular As surface complexes on the freshly formed Fe flocs and compared ferric(III) sulfate (FS) and polyferric sulfate (PFS) performance, and finally provided a practical solution in As-geogenic areas. FS and PFS exhibited a similar As removal efficiency in coagulation and coagulation/filtration in a two-bucket system using 5mg/L Ca(ClO)2. By using the two-bucket system combining coagulation and sand filtration, 500 L of As-safe water (<10 μg/L) was achieved during five treatment cycles by washing the sand layer after each cycle. Fe k-edge X-ray absorption near-edge structure (XANES) and As k-edge extended X-ray absorption fine structure (EXAFS) analysis of the solid residue indicated that As formed a bidentate binuclear complex on ferrihydrite, with no observation of scorodite or poorly-crystalline ferric arsenate. Such a stable surface complex is beneficial for As immobilization in the solid residue, as confirmed by the achievement of much lower leachate As (0.9 μg/L-0.487 mg/L) than the US EPA regulatory limit (5 mg/L). Finally, PFS is superior to FS because of its lower dose, much lower solid residue, and lower cost for As-safe drinking water.
authors.affiliation: State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. Electronic address: jlcuit@163.com.
authors.forename: Jinli
authors.initials: J
authors.lastname: Cui
authors.affiliation: State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. Electronic address: cyjing@rcees.ac.cn.
authors.forename: Chuanyong
authors.initials: C
authors.lastname: Jing
authors.affiliation: School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China.
authors.forename: Dongsheng
authors.initials: D
authors.lastname: Che
authors.affiliation: School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China. Electronic address: zhangjianfeng@xauat.edu.cn.
authors.forename: Jianfeng
authors.initials: J
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
authors.forename: Shuxuan
authors.initials: S
authors.lastname: Duan
mesh_terms: D001151:Arsenic / Q000302:isolation & purification*; D005290:Ferric Compounds / Q000737:chemistry*; D005374:Filtration; D005427:Flocculation; D060587:Groundwater / Q000032:analysis* / Q000737:chemistry; D010084:Oxidation-Reduction; D014874:Water Pollutants, Chemical / Q000302:isolation & purification*; D018508:Water Purification / Q000379:methods*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D005290:Ferric Compounds; D014874:Water Pollutants, Chemical; C024823:ferric sulfate; D001151:Arsenic
keywords: Coagulation; EXAFS; Ferric(III) sulfate (FS); Groundwater As removal; Leaching; Polyferric sulfate (PFS)