Competitive adsorption of arsenic, phosphate and humic substances onto goethite combined with the NOM-CD model...

In natural ecosystems, the (bio)availability of arsenic and phosphorus is greatly controlled by their interactions with metal (hydr)oxides and organic matter. Humic substances (HS), encompassing humic acids (HA) and fulvic acids (FA), constitute the primary form of organic matter. In this study, batch adsorption experiments were conducted and integrated with the NOM-CD model to achieve a molecular-level understanding of HS on the competitive interactions among arsenite, arsenate, and phosphate on goethite surfaces. Results demonstrate that the NOM-CD model is a reliable tool for elucidating the underlying interaction mechanisms of HS, arsenic and phosphate onto goethite. Both HA and FA exhibit a considerable impact on the competitive interactions between arsenite, arsenate, and phosphate at the goethite-water interface, particularly under acidic conditions. At pH below 5.0, influences of HA and FA on the interactions in arsenite-arsenate-phosphate-goethite systems demonstrate notable similarities. The larger particle size of HA results in a greater morphological variation of adsorbed HA (ξ = 5-8), generating not only a stronger steric hindrance effect but also a stronger electrostatic repulsion between the adsorbed HA and oxyanions on goethite. Conversely, the smaller particle size and greater density of carboxylic groups of FA facilitate closer interactions with the goethite surface, thereby reducing the electrostatic potential of goethite even without morphological change (ξ = 1). At pH 5-7, a higher amount of adsorbed HA results in intensified competition between carboxylic groups and oxyanions in comparison with FA, and consequently a higher concentration of arsenite, arsenate, and phosphate in solution. At pH above 7.0, the influence of HS diminishes due to an increase in both hydroxide ions in solution and negative charges on goethite surfaces, reducing the adsorption of arsenate, and phosphate and HS. The NOM-CD model identifies electrostatic repulsions and steric effects as the principal determinants in the complex interactions between multi-oxyanions and HS within goethite systems.