Investigation of the presence of SO₄²⁻ in humic substances extracted from drinking water

Abstract

Humic substances (HS) are a fraction of dissolved organic carbon. There are two main fractions of HSs: humic acid (HA) and fulvic acid (FA). Recent research on HS in water indicates that they can bind with charged species, such as metals and ligands, leading to severe health problems. Further, recent studies have highlighted the increase of manganese (Mn²⁺) levels in ground water in some areas in Sri Lanka, and it is hard to remove manganese by filtration. Therefore, this study focused on the investigation of the presence of SO₄²⁻species bound to HAs in drinking water with and without Mn²⁺. First, the presence of humic substances was confirmed by extracting HA from some selected well water samples in Sri Lanka and characterised using Fourier transform infrared (FTIR) spectroscopy by identifying major functional groups present in the most acceptable model structure of HAs and comparing them with authentic standards. To identify the potential interactions of SO₄²⁻ with HAs, two methods were followed: adding SO₄²⁻ directly from a SO₄²⁻ source and adding through a metal salt. By comparing the FTIR spectra and Raman spectra, the effective binding of SO₄²⁻ with HAs could be proven. Overall, this study was able to prove that SO₄²⁻ binding happens through an elimination/exchange reaction and electrostatic interaction with HAs, and SO₄²⁻ followed by metal binding causes sulphate to bind via the metal.