Experimental tests of cadmium and trace metals adsorption on natural clays and activated carbon from wet phosphoric acid

Abstract

Cadmium and trace metals elements present a significant challenge to the fertilizer industry, considering the increasingly strict regulation on their contents in fertilizers. Consequently, a specific need to develop efficient and cost-effective technologies for their removal has appeared within the last few years. This research article focuses on the adsorption technology already proved for water treatment as a promising and emerging technology for removing cadmium and trace metals from wet phosphoric (WPA), given its ease of use and low cost. Literature highlights a lack of adsorption applications for treating WPA in industrial conditions in terms of P2O5 content and temperature. In this scope, effective natural and activated clays, Montmorillonite (M), Illite (I), and Attapulgite (A), as well as sulfur-impregnated activated carbon, were prepared, characterized, and used for the adsorption of cadmium and trace metals from WPA with 25 % (w/w) P2O5 at 80 °C. For clay adsorbents, different quantities of 1, 2, 3, and 4 % (w/w) were tested with a contact time of 30 min. As a result, it was found that lead was removed in total while there was no significant adsorption of cadmium and nickel. Slight adsorption efficiency of 20 % was registered for chromium and arsenic, equivalent to an adsorption capacities of 2.5 mg/g and 0.2 mg/g, respectively. The latter effect could be attributed to their multiple oxidation states. The chemical activation of clays to pH 1 by using a 5 M NaOH solution increased chromium and arsenic adsorption efficiencies to 46 % and 19 %, equivalent to an adsorption capacities of 0.2 mg/g and 3.7 mg/g, respectively. Adsorption could not be reached for sulfur-impregnated activated carbon despite its large specific surface (528 m2/g) and its sulfur impregnation (10 % w/w). This latter was supposed to activate the chemisorption of cadmium and other trace metals like arsenic, copper, and zinc by forming covalent bonds due to their chemical affinity to sulfur. However, research on the carbon structure showed that sulfur was inactive as it forms a covalent bond with carbon in the mineral structure of activated carbon.