Removal of Pb²⁺ and Cd²⁺ from drinking water using polysaccharide extract isolated from cactus pads (Opuntia ficus indica)

The use of a polysaccharide extract isolated from cactus pads (Opuntia ficus indica) as a low-cost adsorbent for the removal of Pb²⁺ and Cd²⁺ from water was investigated. The Brunauer−Emmett−Teller study showed that the surface area of the extract powder was approximately 2.373 m²/g. Inductively coupled plasma optical emission spectrometry was used to monitor changes in ion concentrations during the adsorption. The adsorption mechanism was studied as a function of physicochemical variables such as initial metal ion concentration, adsorbent dosage, contact time, and pH of the solution. The results indicated that the adsorption percentage of the two cations increased with contact time, attaining equilibrium at 150 and 120 min for Cd²⁺ and Pb²⁺, respectively. At these optimal times, the adsorption capacity of Pb²⁺ and Cd²⁺ increased with increasing pH. Langmuir, Freundlich, Toth, Dubinin–Radushkevich, Sips, and Redlich–Peterson isotherm models were thereafter applied to understand the adsorption mechanism. The qe plot against C_e for the Toth isotherm model had the best fit with low error values and the highest correlation coefficient (R²) values of 0.9973 and 0.9953 for Pb²⁺ and Cd²⁺, respectively. Therefore, the isotherm study reveals that the adsorption mechanism involved is complex because of the presence of different interactions (such as ionic exchange and coagulation), yielding maximum adsorption capacities of 256 and 151 gm/L for Pb²⁺ and Cd²⁺, respectively. Generally, the polysaccharide extract exhibited higher adsorption capacity of Pb²⁺ than of Cd²⁺ under similar conditions. Besides the removal capacities reported in this study, the adsorbent regeneration and its reusability was also investigated.