Characterization and potential use of Djiboutian clays: Adsorption studies for the removal of heavy metal from wastewater

Abstract

The aim of this study was to evaluate the potential use of the two biggest clay deposits from the Djibouti region as raw material for wastewater treatment and for the production of ceramics. The present study is the first to investigate the kinetics, sorption isotherms, hysteresis of Cu and Cd on two clays from the East African Rift System (EARS). Djiboutian clays (DCs) were characterized by a rapid uptake of these heavy metals with an equilibrium time of 400 min and uptake ranging from 48.8% to 97.4% at high initial metal concentrations (15 mg L⁻¹) in the medium. A high adsorption coefficient (k_d^S) values, from 8.31 to 20.80 L g⁻¹, were observed for Cu, indicating a strong affinity of this metal for both clays. The k_d^S values for Cd (0.29–3.64 L g⁻¹) were significantly lower, suggesting different interaction mechanisms. These results are consistent with the Freundlich constant (k_f) values obtained for Cu (8.88–1.05 mg g⁻¹(L·mg⁻¹)^1/n) and Cd (0.80–1.05 mg g⁻¹(L·mg⁻¹)^1/n) sorption isotherms onto DCs clays. Furthermore, desorption process exhibited high hysteresis index (HI) values (2.18–2.33) for Cu, suggesting strong sequestration on both DCs. These sorption behaviors can be associated with the properties of both Djiboutian clays whose characterization revealed large specific surface areas (62.7–64.8 m3/g), high open porosity (44.4–51.2%), and the presence of organic matter and negatively charged clay minerals such as illite, which interact well with divalent metals. Characterization of both clays and the chemical data of DCs plotted on a ternary diagram (SiO₂ ⁻ Al₂O₃−other oxides) suggested that these clays can be applied as raw materials for structural ceramic products. The study underscores the potential of DCs for trapping Cu and Cd pollutants from wastewater and for producing Class BIIb ceramics^.