Biological As^III oxidation and arsenic sequestration onto ZVI-coated sand in an up-flow fixed-bed reactor

A strict authorized Arsenic (As) level (10 μg L⁻¹) in drinking water requires the development of low-cost treatment processes. Biological treatment of As^III (major specie in groundwater) using As^III-oxidizing bacteria (AOB) can be used as an effective pre-oxidation step followed by As removal onto zero valent iron-coated sand. However, the efficiency of As sorption could be affected by a possible development of the bacterial communities in the ZVI-coated sand system. The aims of this work were to study the role of the AOB communities on the filling material (sand or ZVI-coated sand) and to estimate simultaneous biological As^III oxidation and As sorption onto ZVI in a single fixed-bed reactor. Thiomonas arsenivorans, an autotrophic AOB was inoculated in only one of two parallel fixed-bed reactors filled with sand mixed with 1% iron (w_Fe/w_sand). Both reactors were then continuously fed with synthetic water containing 10 mg L⁻¹ of As^III under a high Hydraulic Residence Time (HRT) (4 h) for 23 days and then under lower HRT (1 h) for 10 days. 80% of total As were removed in the non-inoculated reactor under high HRT while only 15% were removed under HRT of 1 h. Hence HRT was a key operating parameter in the fixed-bed system. The inoculated AOB bacterial community could disturb the As sorption under high HRT, while the As removal efficiency under low HRT was better than in the absence of the inoculums. At the end of the experiment, ZVI (and/or its oxide products) was collected at different layers in both reactors. Residual adsorption capacities were evaluated with batch experiments which showed that some layers were not saturated and continued to remove As. PCR-DGGE of aoxB gene was also used to identify the distribution of AOB among these different layers.