Binary fluoride and As(V) adsorption in water using pleco fish bone chars

Abstract

The present work studied individual and binary adsorption of fluorides and As(V) in water on pleco fish bone chars (BC), as well as the effect of BC mass variation on the adsorption capacity of fluoride and As(V) in water for human consumption. The results of individual adsorption indicated that the adsorption of fluoride and As(V) on BC depends on solution pH. The adsorption capacity of fluorides at an initial concentration of 30 mg L^-1 increases approximately 3 times, from 5.9 to 15.3 mg g^-1, when decreasing the pH of the solution from 9 to 5, however, for the case of As(V) an antagonistic effect is observed, the adsorption capacity increases 7 times when raising the pH from 5 to 9, from 18.4 to 132.1 µg g^-1 at an initial As(V) concentration of 300 µg L^-1. Besides, in the binary adsorption, BC showed a higher affinity to adsorb fluoride since its adsorption capacity decreased from 16.55 to 12.50 mg g^-1 as the As(V) concentration increased from 0 to 800 µg L^-1 in solution. In contrast, As(V) adsorption was severely affected, decreasing from 140.2 to 32.7 µg g^-1 when the fluoride concentration in the solution increased from 0 to 100 mg L^-1. On the other hand, in the adsorption of groundwater contaminated with fluoride and As(V), it was determined that increasing the mass of BC from 0.5 to 20 g increases the removal percentage, reaching 99.3 and 75.7% removal for fluoride and As(V), respectively, due to the fact that increasing the mass of the adsorbent leads to a larger area and a greater number of sites that allow the adsorption of these contaminants. The thermodynamic study revealed the spontaneity of fluoride and As(V) adsorption, better affinity for fluoride but higher adsorption rate of As(V) on BC. Characterization techniques such as XRD and EDS allowed identifying hydroxyapatite as the mineral phase of BC, which is responsible for the adsorption of BC. By studying the effect of solution pH on the adsorption capacities and the characterization of BC such as XRD, EDS and TGA, it was determined that the mechanisms of fluoride adsorption are by electrostatic attractions and ion exchange, and for As(V) it is by coprecipitation and ion exchange. It was concluded that BC from pleco fish could be an alternative for treating water contaminated by fluorides and As(V).

Keywords: Arsenic; Binary adsorption; Bone char; Fluorides; Groundwater; Pleco fish.