Column study of chromium(VI) adsorption from electroplating industry by coconut coir pith

The removal of Cr(VI) from electroplating wastewater by coir pith was investigated in a fixed-bed column. The experiments were conducted to study the effect of important parameters such as bed depth (40-60 cm) and flow rate (10-30 ml min−1). At 0.05 Ct/C0, the breakthrough volume increased as flow rate decreased or a bed depth increased due to an increase in empty bed contact time (EBCT). The bed depth service time model (BDST) fit well with the experimental data in the initial region of the breakthrough curve, while the simulation of the whole curve using non-linear regression analysis was effective using the Thomas model. The adsorption capacity estimated from the BDST model was reduced with increasing flow rate, which was 16.40 mg cm−3 or 137.91 mg Cr(VI) g−1 coir pith for the flow rates of 10 ml min−1 and 14.05 mg cm−3 or 118.20 mg Cr(VI) g−1 coir pith for the flow rates of 30 ml min−1. At the highest bed depth (60 cm) and the lowest flow rate (10 ml min−1), the maximum adsorption reached 201.47 mg Cr(VI) g−1 adsorbent according to the Thomas model. The column was regenerated by eluting chromium using 2 M HNO3 after adsorption studies. The desorption of Cr(III) in each of three cycles was about 67-70%. The desorption of Cr(III) in each cycle did not reach 100% due to the fact that Cr(V) was present through the reduction of Cr(VI), and was still in coir pith, possibly bound to glucose in the cellulose part of coir pith. Therefore, the Cr(V) complex cannot be desorbed in solution. The evidence of Cr(V) signal was observed in coir pith, α-cellulose and holocellulose extracted from coir pith using electron spin resonance (ESR).