Biosorption of Cadmium from Aqueous Solutions Using Natural and Treated Pinus halepensis Needles: Batch and Fixed-Bed Studies

This study investigated the adsorption capabilities of three solids derived from dead Pinus halepensis needles, a renewable and low-cost natural material, for the removal of cadmium ions from wastewater: the raw material (P-H2O) and materials chemically modified with NaOH (P-NaOH) and HCl (P-HCl) solutions. The Fourier transform infrared spectra of the three materials revealed the presence of active functional groups, such as hydroxyl and carbonyl groups, which may be involved in the adsorption process. The effects of pH, adsorption kinetics, adsorption isotherms, and the presence of Na+, K+, Ca2+, and Mg2+ ions were studied. Continuous adsorption tests in a fixed-bed column were carried out using the P-H2O biosorbent. The amount of Cd(II) adsorbed increased with increasing pH and followed the order P-NaOH > P-H2O > P-HCl. Cadmium adsorption kinetics were very rapid (less than 30 min in all cases), and the pseudo-second-order kinetic model adequately described the adsorption process. The Sips isotherm model accurately described the adsorption equilibrium and predicted maximum adsorption capacities of 30.50 mg·g−1, 70.11 mg·g−1, and 30.46 mg·g−1 for the P-H2O, P-NaOH, and P-HCl solids, respectively. It was also verified that the amount of cadmium adsorbed decreased substantially in the presence of Na+, K+, Ca2+, and Mg2+ ions. These results highlight that dead pine needles may be a promising, inexpensive, and effective adsorbent for the removal of Cd(II) ions from aqueous solutions.