Adsorption of metal ions onto native and chemically modified agricultural waste

Abstract

The presence of heavy metal ions in the aquatic environment is a matter of great concern to researchers because of their toxic nature, increasing levels of discharge, and other adverse effects of receiving wastewaters. The main objective of the present research is to evaluate the potential of an effective new low-cost adsorbent obtained from agricultural waste, such as cashew nut shell (CNS) and its modified form, i.e., sulphuric acid treated cashew nut shell (STCNS), and to investigate its ability for the removal of metal ions such as Pb(II), Cu(II), Cd(II), Zn(II) and Ni(II) from their aqueous solutions under different experimental conditions, in order to optimize the efficiency of the adsorption process. These two adsorbents were characterized by the Fourier Transform Infra Red (FTIR) and the Scanning Electron Microscopy (SEM) analysis, and the results show that the STCNS has good surface properties than the CNS for the removal of metal ions from their aqueous solutions. Batch adsorption experiments were carried out using the CNS and STCNS adsorbents by varying the solution pH, the adsorbent dose, contact time, initial metal ion concentration, and temperature, in a rotary shaker. The results show that adsorption of metal ions onto the adsorbents was spontaneous and exothermic in nature. Desorption was attempted with the chemical regeneration technique, to study metal ion desorption from the spent adsorbents. The results show that with an increase in the HCl concentration, there is an increase in desorption, and it reaches a constant value with 0.15 M HCl for the spent CNS, and 0.3 M HCl for the spent STCNS. The amount of metal ions desorbed was high (95%), which indicates that there is a weak attraction between the adsorbent and metal ion, and hence physisorption seems to be the operating adsorption process. This implies that these adsorbents can be used once again, as adsorbents for wastewater treatment. newline