Equilibrium and kinetic studies on adsorption of chromium(VI) onto pine-needle-generated activated carbon

Chromium is extensively used in a large number of industries and is often discharged through the wastewater effluents to pollute water sources. Being a heavy metal, it invariably leads to serious health risks when ingested. The aim of the present study is to test a readily available low-cost precursor, dry pine needles, for the production of activated carbon and determine its efficiency in removing Cr(VI) from water. Process parameters such as efficiency of metal removal, capacity of the activated carbon, pH, and concentration of adsorbate were investigated. The characterization of the adsorbent was performed using scanning electron microscopy and X-ray diffraction. The point of zero charge was determined. Both batch and column adsorption experiments were conducted. Adsorption equilibrium isotherms as well as adsorption kinetics were generated from batch experiments. Breakthrough curves were generated to assess adsorption capacities using column adsorption tests. The results obtained from the batch tests showed that the Freundlich as well as Temkin isotherms constituted a better fit for the adsorption data than the Langmuir isotherm. Maximum adsorption capacity of pine-needle activated carbon for Cr(VI) was 65.36 mg g−1. Furthermore, the adsorption kinetics followed a pseudo-second order which confirms chemisorption to be the mechanism responsible for the removal of Cr(VI) using pine-needle activated carbon as adsorbent. The results of the column tests indicated that the highest metal removals were achieved at lower initial concentrations, while the highest adsorption capacities were achieved at the initial concentration of 20 mg L−1. The results also depicted that Cr(VI) is almost exclusively removed at low pH values (pH 4 being the lowest pH tested) with removals decreasing appreciably with the increase in pH.