Abstract
Due to the importance of adsorption kinetics and redox transformation of arsenic (As) during the adsorption process, the present study elucidated natural organic matter (NOM) effects on As adsorption-desorption kinetics and speciation transformation. The experimental procedures were conducted by examining interactions of arsenate and arsenite with different concentrations of humic acid (HA) as a model representative of NOM, in the presence of iron oxide based adsorbent (IBA), as a model solid surface in three environmentally relevant conditions, including the simultaneous adsorption of both As and HA onto IBA, HA adsorption onto As-presorbed IBA, and As adsorption onto HA-presorbed IBA. Experimental adsorption-desorption data were all fitted by original and modified Lagergren pseudo-first and -second order adsorption kinetic models, respectively. Weber’s intraparticle diffusion was also used to gain insight into the mechanisms and rate controlling steps, which the results suggested that intraparticle diffusion of As species onto IBA is the main rate-controlling step. Different concentrations of HA mediated the redox transformation of As species, with a higher oxidation ability than reduction. The overall results indicated the significant effect of organic matter on the adsorption kinetics and redox transformation of As species, and consequently, the fate, transport and mobility of As in different environmentally relevant conditions.
Highlights
Arsenic (As) is widely recognized as a toxic carcinogen and a threat to the world’s water resources [1,2].Exposure through drinking water, food or air causes different adverse health effects, which depend on the level of exposure and on the As speciation [3,4]
Arsenic adsorption-desorption kinetics and aqueous speciation were evaluated in three types of equilibrated ternary systems consisting of As, humic acid (HA) and iron oxide based adsorbent (IBA)
The first system was composed of the co-presence of both As and HA interacting with IBA (As-HA-IBA)
Summary
Arsenic (As) is widely recognized as a toxic carcinogen and a threat to the world’s water resources [1,2].Exposure through drinking water, food or air causes different adverse health effects, which depend on the level of exposure and on the As speciation [3,4]. The fate and bioavailability of As and many other heavy metals in contaminated environments are mainly controlled by adsorption reactions on the soil minerals’ surfaces [6]. Many adsorbent media in aquatic environments are known to have a strong reaction with dissolved substances affecting the transfer of many pollutants, including As [7]. Iron oxides are relatively abundant in natural systems, such as soils, rocks and ground water [8] and are known to have a strong affinity for As in water due to their high surface areas and partially positive charges [9]. Different solids coated with iron oxides have been used in adsorption processes, including zeolite [10], montmorillonite [11], cement [12], activated carbon [13], and sand [14]. Adsorption and desorption reactions between As and iron-oxide surfaces are important due to the widespread presence of iron oxides in the hydro-geologic environment as coatings on other solids result in a considerable adsorption of As to iron-oxide surfaces [15,16]
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