Removal of Cr(VI) from synthetic wastewater by adsorption onto coffee ground and mixed waste tea.

We attempted to recycle mixed waste tea and coffee ground as alternative low-cost adsorbents for Cr(VI) removal. The adsorption parameters optimized were: initial Cr(VI) concentration (10-30 mg L-1), contact time (180 min), adsorbent dose (2.0 g L-1), initial pH (2.0), temperature (30-50 °C), and agitation speed (250 rpm). Freundlich isotherm was found better fitted with a high correlation coefficient (R2 = 0.97 for mixed waste tea and 0.92 for coffee ground) than to Langmuir model (R2 = 0.89 for mixed waste tea and 0.86 for coffee ground) for the 10-250 mg L-1 concentration range. Analysis of kinetic studies indicated that Cr(VI) adsorption by both adsorbents was consistent with the pseudo-second-order kinetic model with a good R2 and Marquardt's present standard deviation (MPSD) values. Experimental data demonstrated a sorption capacity of 94.34 mg g-1 of mixed waste tea and 87.72 mg g-1 of coffee ground. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Energy dispersive X-ray spectroscopy (EDS) revealed the noticeable chromium accumulation on the adsorbent surfaces after adsorption. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) studies showed that carbon and oxygen functional groups on the surface of both adsorbents involved in Cr(VI) adsorption. The adsorbents could be reused four times. Large-scale operation using 100 L of packed-bed reactor showed the breakthrough time of adsorption for mixed waste tea of 30 min in 100 mg L-1 Cr(VI) concentration. These results suggested that mixed waste tea and coffee ground be considered as alternative adsorbent for Cr(VI) removal.