IntroductionMining, electroplating, and metal processing are the main sources of heavy metal contamination. Metals such as lead, arsenic, chromium, mercury, copper, and zinc have been characterized as hazardous heavy metals. Due to their hazardous effects, persistency, and tendency to accumulate, effective removal of heavy metal ions from wastewater is an important issue.Various agricultural by-products have been used to remove heavy metals from solutions. Using waste materials as low-cost adsorbents is attractive as it reduces costs for their disposal. Several studies have employed low-cost adsorbents such as orange peels [1] [2] were reported for adsorption onto coffee residues. However, only pseudo first-and second-order models were tested for adsorption kinetics in these investigations. Generally, an adsorption process can be characterized as three stages:(i) external diffusion or boundary-layer diffusion, (ii) intraparticle mass diffusion, (iii) adsorption on interior sites.Pol. J. Environ. Stud. Vol. 24, No. 2 (2015), [761][762][763][764][765][766][767] Original Research
AbstractUntreated coffee residues (UCRs) were used to remove heavy metal ions from aqueous solutions. The effects of solution pH, heavy metal ions, and UCR concentrations on heavy metal ion removal by UCRs were determined. The removal percentage for heavy metal ions increased as pH and UCR doses increased; conversely, it decreased as the concentration of heavy metal ions increased. This study employed the pseudo firstand second-order kinetics models, the intraparticle diffusion model, and the Bangham model to simulate adsorption kinetics of Pb 2+ and Zn 2+ onto UCRs. Simulation results indicate that adsorption kinetics fit well with the pseudo second-order model. After reaction for 180 min at [adsorbate] = 20 mg/L, [UCRs] = 2 g/L, and pH 5, the adsorption percentage of Pb 2+ and Zn 2+ was 96% and 44%, respectively; moreover, the adsorption density of Pb 2+ and Zn 2+ onto UCRs was 9.7 and 4.4 mg/g (4.7×10 -2 and 6.8×10 -2 mmol/g), respectively. The adsorption of Pb 2+ and Zn 2+ onto UCRs is controlled mainly by surface diffusion.
