Abstract
Arsenic (As)-laden wastewater may pose a threat to biodiversity when released into soil and water bodies without treatment. The current study investigated the sorption properties of both As(III, V) oxyanions onto iron hydroxide (FHO) by chemical coagulation. The potential mechanisms were identified using the adsorption models, ζ-potential, X-ray diffraction (XRD) and Fourier Transform Infrared Spectrometry (FT-IR) analysis. The results indicate that the sorption kinetics of pentavalent and trivalent As species closely followed the pseudo-second-order model, and the adsorption rates of both toxicants were remarkably governed by pH as well as the quantity of FHO in suspension. Notably, the FHO formation was directly related to the amount of ferric chloride (FC) coagulant added in the solution. The sorption isotherm results show a better maximum sorption capacity for pentavalent As ions than trivalent species, with the same amount of FHO in the suspensions. The thermodynamic study suggests that the sorption process was spontaneously exothermic with increased randomness. The ζ-potential, FT-IR and XRD analyses confirm that a strong Fe-O bond with As(V) and the closeness of the surface potential of the bonded complex to the point of zero charge (pHzpc) resulted in the higher adsorption affinity of pentavalent As species than trivalent ions in most aquatic conditions. Moreover, the presence of sulfates, phosphates, and humic and salicylic acid significantly affected the As(III, V) sorption performance by altering the surface properties of Fe precipitates. The combined effect of charge neutralization, complexation, oxidation and multilayer chemisorption was identified as a major removal mechanism. These findings may provide some understanding regarding the fate, transport and adsorption properties onto FHO of As oxyanions in a complex water environment.
Highlights
The emission of arsenic (As)-contaminated wastewater from industries, mining sites and smelting areas into fresh water bodies has raised serious concerns owing to its harmful effects on human health and its destruction of the surrounding environment [1,2]
Such results are in good agreement with the fact that the first dissociation constant of the H3AsO3 species is 9.22, and As(III) acts as a Lewis base, it can interact with Lewis acid, i.e., FHO precipitates across a wide pH range [21,22]
Equilibrium sorption for trivalent and pentavalent As was achieved within 20 min of the flocculation time, with qe of 7.996 and 9.685 g/mol, respectively (Table 1). These findings suggest that the sorption rate and capacity of pentavalent As on FHO are better than the trivalent species
Summary
The emission of arsenic (As)-contaminated wastewater from industries, mining sites and smelting areas into fresh water bodies has raised serious concerns owing to its harmful effects on human health and its destruction of the surrounding environment [1,2]. Among the four oxidation states of As, the pentavalent form (As(V)) predominates in aerobic environments, whereas the trivalent form (As(III)) occurs in reduced conditions, yet their chemistry, toxicity and sorption affinity are markedly different [3]. Various environmental agencies, including the Pakistan Environmental Protection Agency (Pak-EPA), the United States Environmental Protection Agency (USEPA) and the World Health Organization (WHO), have set the regulation limit for As at 10 μg/L [4]. As such, it would be critical, both scientifically and technologically, to develop highly efficient methods for removing both As species from water
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