Immobilizing arsenic in anoxic groundwater by iron and nitrate treatment: divergence driven by reactive organic carbon

Li, Zengyi (2024) Immobilizing arsenic in anoxic groundwater by iron and nitrate treatment: divergence driven by reactive organic carbon. Doctoral thesis, University of East Anglia.

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Groundwater arsenic (As) of natural origin is a global issue affecting the health of an estimated 100-200 million people in >70 countries. In situ remediation has proven to be challenging due to the unstable nature of iron (Fe) oxide in anoxic groundwater. Efforts made to stimulate minerals (trans)formations in reducing groundwater for arsenic immobilization via iron and nitrate augmentation in laboratory and field experiments have shown promise. However, the mechanisms governing the formation of stable minerals to sequester arsenic are still poorly understood. While laboratory studies overwhelmingly favor the formation of mixed Fe(II)-Fe(III) oxides such as nano-magnetite for arsenic immobilization, field observations suggest the likely significant role of sulfur (S) minerals. This study suggests a divergence driven by reactive organic carbon (ROC), if indeed the processes regulating Fe/S mineral (trans)formation in oligotrophic groundwater are microbially mediated, aiming to further understand the controlling mechanisms and improve the treatment approach.

To ascertain the likely complex combinations of Fe and/or S minerals responsible for arsenic sequestration in anoxic groundwater systems, experiments were conducted in the field kept under anoxic conditions, using 8 columns packed with freshly collected aquifer sediment from a depth of 10 m and consisted of fine sand with 10.6 mg/kg arsenic and 0.36% total organic carbon. The column experiments lasted over 2 months. Pre-treatment equilibrium took 11 days. All columns were fed with fresh natural groundwater containing 89±6 μg/L As(III) (109±9 μg/L total-As). The treatment took 10 days. In addition to 2 control columns characterized by 0.79±0.07 mmol/L of influent sulfate, the experiments included 2 columns each for (a) reactive organic carbon (ROC) amendments (1 mmol/L acetate/lactate), and with/without (b) 5 mmol/L Fe(II)-nitrate supplementation. Post treatment, all columns returned to groundwater-fed only and lasted 50 days.

Fe(II)-nitrate treatments nearly doubled absorbed arsenic in sediments, with phosphate extractable arsenic increasing from 0.75 mg/kg to 1.13±0.27 mg/kg, equivalent to accumulation of 0.11 μmol of this “strongly-absorbed” arsenic, while sequestering 1.68±0.02 μmol arsenic based on influent-effluent mass balance. The wide difference, together with 0.56 μmol arsenic associated with acidic-volatile-sulfide, suggest that sulfate reduction resulted in precipitation of sulfide minerals as a major arsenic sink. ROC-amendment alone enhanced arsenic mobilization, with 0.12 and 0.22 μmol arsenic released from solid to aqueous phase during acetate- and lactate-amendment, respectively. In contrast, lactate-amendment combined with Fe(II)-nitrate treatment increased HCl-extractable Fe(III) in sediments from 88 to 125 mg/kg, while inhibited sulfide precipitation, especially increased 25% phosphate-extractable “strongly-absorbed” As. Laboratory column experiments using artificial groundwater with lactate also found lower sulfide precipitation after Fe(II)-nitrate treatment. Lactate coupled to Fe(II)-nitrate treatment, or acetate alone, increased reactive Fe(III) in sediments, and inhibited sulfate reduction.

Microbial community obtained by 16s rRNA sequencing exhibited dramatic changes in all sediment samples collected at the end of the experiments compared to the initial sediment. Significant enrichment in nitrate-dependent iron-oxidizers were evident in end-point column sediments for Fe(II)-nitrate augmentation, and especially so with additional lactate-amendment. Interestingly, lactate-amendment alone significantly enriched sulfate reducers, but acetate-amendment did not. Consistent with the aforementioned chemistry changes, ROC amendment combined with Fe(II)-nitrate treatment resulted in lower abundance of sulfate reducers, and may contributed to the lack of sulfide precipitation observed in these columns.

In conclusion, by shedding light on coupled biogeochemical cycles of arsenic, iron and sulfur, the findings are vital for the development of effective in situ mitigation technology which calls for augmentation of ROC together with iron and nitrate.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Nicola Veasy
Date Deposited: 26 Jun 2024 13:22
Last Modified: 26 Jun 2024 13:22


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