Aqueous cadmium removal by hydroxylapatite and fluoroapatite

Abstract

Reducing the bioavailability of toxic heavy metals in groundwaters and urban soils by phosphate addition is an effective technique described in the literature. It is based on the reaction between metal ions and phosphates and results in the precipitation of metal substituted phosphate phases. The formed phosphates are highly insoluble and thermodynamically stable over almost entire pH and Eh range. In the presented study the efficiency and mechanism of cadmium uptake by synthetic hydroxylapatite and natural fluoroapatite was examined within the pH range of 3 7 for different reaction times (2 1440 hours). The solids after reactions were characterized by XRD and SEM-EDS. Percentage reduction of cadmium concentration in the experiments with fluoroapatite and hydroxylapatite, regardless of pH, did not exceed 17% and 25%, respectively. Cadmium uptake from the solution mainly resulted from the formation of cadmium phosphates and/or Ca-Cd phosphate solid solutions on the apatites surface. The release rate of phosphate ions by hydroxylapatite was relatively high. This promoted crystallization of a large number of small crystals. In turn dissolution of fluoroapatite was slower and thus the formation of large crystals was observed. There was no clear evidence for cadmium-calcium ion-exchange mechanism.

Influence of natural minerals on contaminated solutions pH values

“…Matusik et al (Matusik, 2012) determined that hydroxy apatite affects the reduction of Cd content in the solution at different pH values of the basic solution 3, 5 and 7 and leads to an increase in the pH value of the basic solutions to 6.07, 6.90 and 7.21, respectively. In the experiment with natural apatite, the initial pH values were slightly changed to 3.59, 6.45 and 6.90.…”

Synthesis and characterization of cadmium chlorapatite Cd5(PO4)3Cl

“…The removal of cadmium from the environment is important and various treatment methods such as precipitation, adsorption, ion exchange, and membrane filtration have been used to remove to achieve this (Deng et al 2017;Lee et al 2018;Wong et al 2014). Immobilization by minerals such as apatite is a promising method for cadmium removal (Matusik et al 2012).…”

“…Many studies addressed the interaction of cadmium with apatites, especially the sorption of cadmium by apatites (Klee, Engel 1970;Jeanjean et al 1996;Corami et al 2008;Matusik et al 2012;Zhu et al 2017). Possible mechanisms for the sorption process are ion exchange, superficial sorption, and precipitation (Jeanjean et al 1996).…”

Experimental immobilization of Zn, Pb and Cd by additives to highly contaminated soils

“…Experimentally calculated solubility constants of selected trace metal phosphates and arsenates are relatively low and range from 9.0 x 10 -33 for zinc phosphate (Zai et al 2020), 10 -65.58 for cadmium chlorapatite (Wołowiec et al 2019), 10 -76.11 for lead arsenate, to 10 -79.6 for lead chlorapatite (pyromorphite) (Flis et al 2011). Furthermore, these phases can be highly stable over a wide range of pH and Eh conditions (Matusik et al 2012). It is noted that the efficiency of treatment is affected by various parameters including initial pH and ion concentration, and the products vary in crystallite size and morphology (Zhang et al 1997;Manecki et al 2006;Matusik et al 2008;Zhu et al 2016;Shi et al 2020).…”