Role of organic matter and humic substances in the binding and mobility of arsenic in a Gangetic aquifer

Abstract

Arsenic (As) enrichment in groundwater has led to extensive research, particularly on the factors responsible for its release into groundwater. In the Gangetic plain, organic matter driven microbial reduction of Fe-oxyhydroxides is considered as the most plausible mechanism of As release into groundwater. However, the role of organic matter in the aqueous environment is not well known and particularly that of organometallic complex. In this study, we have characterized bulk sediment and groundwater samples, collected from Barasat, West Bengal, India, to understand the effect of organic matter in the binding and mobility of As in the subsurface environment. The results showed a moderate correlation (R² = 0.49, p < 0.05) between dissolved organic carbon (DOC) and As in groundwater, suggesting that DOC has a role in releasing As into groundwater. The relative fluorescent intensity (RFI) of the dissolved humic substances in groundwater showed a maximum value of 65 QSU (mean: 47 ± 8 QSU). FT-IR spectra of the extracted humic acid fractions of the sediment showed COO-, C = O, OH, and C = C (aromatic ring) functional groups, which may act as a chelating agents with the metal(loid)s. FT-IR spectra of the HA-As complex exhibited specific peaks at 1242 and 832 cm⁻¹ in the fingerprint region. This is similar to the extracted humic acid fractions of the Gangetic sediment, suggesting binding of As with humic substances.

Keywords:

• Arsenic-enriched aquifer
• FT-IR spectra
• Gangetic aquifer
• humic substances
• organic matter

Acknowledgments

This work was supported by the National Science Council of Taiwan (Project No: NSC 97-2627-M-006-003).