Interaction of Arsenic Species with Organic Ligands: Competitive Removal from Water by Coagulation-Flocculation-Sedimentation (C/F/S)

Abstract

The co-occurrence of arsenic (As) and organic ligands in water bodies has raised environmental concerns due to their toxicity and adverse effects on human health. The present study aims to elucidate the influences of hydrophobic/hydrophilic organic ligands, such as humic acid (HA) and salicylic acid (SA), on the interactive behavior of As species in water. Moreover, the competitive removal behaviors of As(III, V) species and total organic carbon (TOC) were systematically investigated by coagulation-flocculation-sedimentation (C/F/S) under various aqueous matrices. The results showed the stronger binding affinity of As(V) than As(III) species, with a higher complexation ability of hydrophobic ligands than hydrophilic. The media containing hydrophilic ligands require smaller ferric chloride (FC) doses to achieve the higher As(III, V) removal, while the optimum FC dose required for As(III) removal was found to be higher than that for As(V). Moreover, hydrophobic ligands showed higher TOC removal than hydrophilic ligands. The pronounced adverse effect of a higher concentration of hydrophobic ligands on the removal efficiencies of As(V) and TOC was observed. The adsorption of As(V) on Fe precipitates was better fitted with the Langmuir model but the Freundlich isotherm was more suitable for As(III) in the presence of hydrophilic SA. Moreover, TOC removal was substantially decreased in the As(V) system as compared to the As(III) system due to the dissolution of Fe precipitates at higher As(V) concentrations. The results of FC composite flocs demonstrated that the combined effect of oxidation, charge neutralization and adsorption played an important role in the removal of both toxicants during the C/F/S process. In summary, the findings of the present study provide insights into the fate, mobility and competitive removal behavior of As(III, V) species and organic ligands in the water treatment process.

Keywords: adsorption; arsenic; coagulation-flocculation-sedimentation (C/F/S); competitive removal; interaction; organic ligands; water treatment.

Figure 1

Percentages of bound (A) As(III) and (B) As(V) in 10 mgL⁻¹ of humic/salicylic acid solutions under neutral pH (7 ± 0.1) at 25 ± 1 °C.

Figure 2

Under various ferric chloride (FC) doses, removal efficiencies of (A) As(III) and (B) As(V) and respective total organic carbon (TOC) from solution.

Figure 3

At various organic ligands concentrations, removal efficiencies of (A) As(III) and (B) As(V) and respective TOC from solution.

Figure 4

Adsorption isotherm of As adsorption onto precipitated Fe in (A,B) As(III); and (C,D) As(V) system in the presence of 10 mgL⁻¹ organic ligands under varying initial As concentration (0–10 mgL⁻¹) at pH 7.0 ± 0.1.

Figure 5

Under various initial As(III, V) concentrations (0–10 mgL⁻¹), TOC removal in (A) As(III) and (B) As(V) system in the presence of 10 mg L⁻¹ organic ligands (HA/SA).

Figure 6

Fourier transform infrared (FT-IR) spectra of (A) As(III, V) species after interaction with organic ligands and (B) FC composite flocs after the C/F/S process.