Interplay of S and As in Mekong Delta sediments during redox oscillations

• Published in: Di xue qian yuan. Vol. 10; no. 5; pp. 1715 - 1729
• Main Authors: Phan, Van T.H., Bardelli, Fabrizio, Le Pape, Pierre, Couture, Raoul-Marie, Fernandez-Martinez, Alejandro, Tisserand, Delphine, Bernier-Latmani, Rizlan, Charlet, Laurent
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier B.V 01.09.2019; Elsevier Science Ltd; Ho Chi Minh City University of Technology(HCMUT),Vietnam National University,Ho Chi Minh City(VNU-HCM),268 Ly Thuong Kiet,District 10,Ho Chi Minh City,Viet Nam%Institute of Nanotechnology(CNR-Nanotec),00186,Rome,Italy%Institut de Mineralogie,de Physique des Materiaux et de Cosmochimie(IMPMC),UMR 7590 CNRS-UPMC-IRD-MNHN,4 Place Jussieu,75252,Paris cedex 05,France%Département de chimie,Université Laval,1045 Avenue de la Médecine,Québec,QC,G1V 0A6,Canada%University Grenoble Alpes,CNRS,IRD,IFSTTAR,ISTerre,38000,Grenoble,France%Ecole Polytechnique Fédérale de Lausanne(EPFL)-Environmental Microbiology Laboratory(EML),EPFL-ENAC-IIE-EML,Station 6,CH-1015,Lausanne,Switzerland; University Grenoble Alpes,CNRS,IRD,IFSTTAR,ISTerre,38000,Grenoble,France; Elsevier
• Subjects: Air injection; Arsenic; Bioreactors; Chemical reactions; Compressed air; Cycles; Earth Sciences; Floodplains; Geochemistry; Groundwater; Gypsum; Gypsum dissolution; Isotope oxygen; Isotope sulfur; Organic chemistry; Oxidation; Pyrite; Pyrite oxidation; Redox oscillation; Redox reactions; Sciences of the Universe; Sediments; Sparging; Speciation; Gypsum dissolution; Redox oscillation; Isotope sulfur; Isotope oxygen; Pyrite oxidation
• ISSN: 1674-9871; 2588-9192
• DOI: 10.1016/j.gsf.2018.03.008

The cumulative effects of periodic redox cycling on the mobility of As, Fe, and S from alluvial sediment to groundwater were investigated in bioreactor experiments. Two particular sediments from the alluvial floodplain of the Mekong Delta River were investigated: Matrix A (14 m deep) had a higher pyrite concentration than matrix B (7 m deep) sediments. Gypsum was present in matrix B but absent in matrix A. In the reactors, the sediment suspensions were supplemented with As(III) and SO42−, and were subjected to three full-redox cycles entailing phases of nitrogen/CO2, compressed air sparging, and cellobiose addition. Major differences in As concentration and speciation were observed upon redox cycling. Evidences support the fact that initial sediment composition is the main factor controlling arsenic release and its speciation during the redox cycles. Indeed, a high pyrite content associated with a low SO42− content resulted in an increase in dissolved As concentrations, mainly in the form of As(III), after anoxic half-cycles; whereas a decrease in As concentrations mainly in the form of As(V), was instead observed after oxic half-cycles. In addition, oxic conditions were found to be responsible for pyrite and arsenian pyrite oxidation, increasing the As pool available for mobilization. The same processes seem to occur in sediment with the presence of gypsum, but, in this case, dissolved As were sequestered by biotic or abiotic redox reactions occurring in the Fe–S system, and by specific physico-chemical condition (e.g. pH). The contrasting results obtained for two sediments sampled from the same core show that many complexes and entangled factors are at work, and further refinement is needed to explain the spatial and temporal variability of As release to groundwater of the Mekong River Delta (Vietnam). [Display omitted] •Inhibition of microbial sulfate reduction due to low pH (3–4).•A high pyrite-rich sediment resulted mainly in an increase in As(III)(aq) during anoxia; whereas a decrease in As(V)(aq) during oxidation.•A lower pyrite content and containing gypsum resulted As sequestration during the cumulative effects of redox cycling.•Oxidation and low pH (3–4) lead to As(V) co-precipitations with Fe (oxyhydr)oxides and/or Fe oxyhydroxysulfate.