Behavior of arsenic and geochemical modeling of arsenic enrichment in aqueous environments |
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| Institution: | 1. School of Water and Environment, Chang''an University, Xi''an 710054, Shaanxi, China;2. Key Laboratory of Subsurface Hydrology and Ecological Effect in Arid Region of the Ministry of Education, Chang''an University, Xi''an 710054, Shaanxi, China;3. State Key Laboratory of Subtropical Building Science, South China University of Technology, Guangzhou, China;4. South China Institution of Geotechnical Engineering, School of Civil Engineering and Transportation, South China University of Technology, Guangzhou, China;5. State Key Laboratory of Frozen Soil Engineering, Northwest Institute of Eco-Environmental and Resources, CAS, Lanzhou 730000, China;1. School of Environmental Studies, China University of Geosciences, 430074 Wuhan, China;2. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 430074 Wuhan, China;3. School of Environmental Science and Engineering, Southern University of Science and Technology, 518055 Shenzhen, Guangdong, China;4. Geological Survey, China University of Geosciences, 430074 Wuhan, China;1. School of Earth and Environment, University of Leeds, LS2 9JT Leeds, UK;2. Boone Pickens School of Geology, Oklahoma State University, Stillwater, OK, USA;3. Centre for Environmental and Marine Studies, University of Aveiro, Portugal;4. Department of Geosciences, University of Oslo, Oslo, Norway;5. Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany;6. MARUM - Center for Marine Environmental Sciences and Department of Geosciences, University of Bremen, Germany |
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| Abstract: | Arsenic is present in aqueous environments in +III and +V oxidation states. In oxidizing environments, the principle attenuation mechanism of As migration is its adsorption on Fe(III) oxide and hydroxides. The adsorption affinity is higher for As(V) under lower pH conditions and for As(III) under higher pH conditions. Ferric oxide and hydroxides can dissolve under low Eh and pH conditions releasing adsorbed As. Oxidation-reduction processes often involve high organic matter content in sediments and also contamination by organics such as BTEX. Arsenic may desorb under high pH conditions. Changes of pH can be related to some redox reactions, cation exchange reactions driving dissolution of carbonates, and dissolution of silicates. In very reducing environments, where SO4 reduction takes place, secondary sulfide minerals like As-bearing pyrite and orpiment, As2S3, can incorporate As. Geochemical modeling can be divided into two principal categories: (a) forward modeling and (b) inverse modeling. Forward modeling is used to predict water chemistry after completion of predetermined reactions. Inverse modeling is used to suggest which processes take place along a flowpath. Complex coupled transport and geochemistry programs, which allow for simulation of As adsorption, are becoming available. A common modeling approach is based on forward modeling with surface complexation modeling (SCM) of As adsorption, which can incorporate the effect of different adsorbent/As ratios, adsorption sites density, area available for adsorption, pH changes and competition of As for adsorption sites with other dissolved species such as phosphate. The adsorption modeling can be performed in both batch and transport modes in codes such as PHREEQC. Inverse modeling is generally used to verify hypotheses on the origin of As. Basic prerequisites of inverse modeling are the knowledge of flow pattern (sampling points used in model have to be hydraulically connected) and information about mineralogy including As mineral phases. Case studies of geochemical modeling including modeling of As adsorption are presented. |
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