Adsorption of Chromium (VI) on Calcium Phosphate: Mechanisms and Stability Constants of Surface Complexes

Abstract

Abstract:The adsorption of chromate on octacalcium phosphate (OCP) was investigated as a function of contact time, surface coverage, and solution pH. The ion exchange method was adapted to establish the interaction mechanism. Stoichiometry exchange of H + /OH − was evaluated at a pH range of 3-10, and obtained values ranged between 0.0 and 1.0. The surface complexes formed between chromate and OCP were found to be > S(HCrO 4 ) and > S(CrO 4 ). The logarithmic stability constant logK 1-1 , and the logK 10 values of the complexes, were 6.0 in acidic medium and 0.1 in alkaline medium, respectively. At low pH and low surface coverage, the bidentate species > S(HCrO 4 ) 2 with logK 10.5 of about 2.9, was favored at a hydration time of less than 150 min. The contribution of an electrostatic effect to the chromium uptake by the OCP sorbent, was also evaluated. The results indicate that the adsorption of chromate on OCP is of an electrostatic nature at a pH ≤ 5.6, and of a chemical nature at a pH > 5.6.

Removal of Cr(VI) from Waters by Multi-Walled Carbon Nanotubes: Optimization and Kinetic Investigations

“…The Freundlich model is an empirical expression describing adsorption onto heterogeneous surfaces, having the adsorbent surface sites with a variation of binding energies. In this case, the equation also in its linear form is ln [ Cr ] c,e = ln k f + 1 __ n ln [ Cr ] c,e (8) where k f and n are parameters related to the Freundlich model.…”

“…The data of the amount of chromium(VI) adsorbed on the MWCNTs (mg/g) and the metal concentration remaining in solution (mg/L) are fitted to the Langmuir and Freundlich models represented by Eqs. (7) and (8), respectively. The relative parameters obtained from the fit are listed in Table 5.…”

“…Despite its toxic character, chromium(VI) is widely used in various industries, being its recovery from the corresponding liquid effluents a primary target before their discharge to natural waters. Several technologies have found application to remove and/or recover chromium(VI) from these process wastes: pseudo-emulsion strip dispersión pertraction [1,2], adsorption onto activated carbons [3], liquidliquid extraction [4], biomass adsorption [5,6], adsorption onto natural zeolites [7], adsorption onto phosphates [8], ion exchange [9] and electro-assisted and photo-assisted technologies [10]. Among them, adsorption onto carbon nanotube (CNT) technology could be competitive when the metal is present at low concentrations in the aqueous solution.…”

Special Issue on Wastewater Treatment and Reuse Technologies

“…Included in this special issue is another article on heavy metal remediation from industrial wastewater by Elyahyaoui et al [22]. Phosphate materials, particularly calcium phosphate, have been studied as effective adsorbents for heavy metals from wastewaters and polluted soils.…”

“…However, there is a lack of understanding of the mechanisms of heavy metal immobilization that involves protonation/deprotonation of surface hydroxyl groups and their interaction with the metal oxyanion. Therefore the article entitled "Adsorption of Chromium (VI) on Calcium Phosphate: Mechanisms and Stability Constants of Surface Complexes" [22] investigated the mechanisms of complexation of hexavalent chromium with low-crystallized octacalcium phosphate. The obtained results suggested that Cr (VI) adsorption on octacalcium phosphate was of an electrostatic nature in acidic solutions, and of a chemical nature in lower acidic to alkaline solutions.…”