Adsorption behaviour of Cr(VI) onto macro and micro-vesicular volcanic rocks from water

Adsorption by volcanic rocks is investigated as a possible alternative to the conventional method of Cr(VI) removal from polluted waters. In this work, adsorption of Cr(VI) onto pumice (VPum) and scoria (VSco) has been studied by using a batch method at room temperature. The following factors affecting sorption of Cr(VI) were investigated: solution pH, contact time, type and size of adsorbents, adsorbent dose and initial metal ion concentration. The maximum adsorption yield, 77% for VSco and 80% for VPum, was obtained at low pH of about 2. The applicability of the Langmuir as well as Freundlich adsorption isotherms for the present system is tested. The batch sorption kinetics has been mathematically described using the Lagergren pseudo-first order, pseudo-second order equations and equations for intra-particle and liquid film diffusion. For both VPum and VSco, the entire kinetic data fitted well with pseudo-second-order reaction rate model. In the case of VSco, the rate constant was the highest (338 × 10−3 kg mg−1 h−1) and the VPum gave the lowest (7 × 10−3 kg mg−1 h−1). The experimental results inferred that electrostatic attraction and surface complexation are the major adsorption mechanisms for binding Cr(VI) ions to the macro and micro-vesicular volcanic rocks. The two volcanic rocks tested have potential for an inexpensive and suitable method for removal of Cr(VI) from polluted waters.

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► Adsorption of Cr(VI) on macro and micro-vesicular rocks increased markedly at low pH conditions.
► The kinetics rate constant decreases with the particle size.
► The processes of Cr(VI) removal on the adsorbents were found to be complex.
► The rocks have potential for an inexpensive and suitable method for purification Cr(VI)-bearing waters.