Green walnut shell as a new material for removal of Cr(VI) ions from aqueous solutions

The removal of aqueous hexavalent chromium (Cr(VI)) by green walnut shell (GWS) was investigated in batch experiments. GWS is a readily available and low-cost horticulture solid waste. The efficiency of this biomass for Cr(VI) removal as impacted by various operational parameters was characterized. Tested parameters are: initial solution pH (pH(0)), initial GWS mass loading ([GWS] (g L- 1)), contact time (t (min)), GWS particle size (d (mu m)), initial Cr(VI) concentration ([Cr(VI)](0) (mg L- 1)), and ionic strength (I(M)). Results revealed that Cr(VI) chemical reduction (to Cr(III)) followed by adsorption or surface precipitation is the removal mechanism. The optimal operational conditions for Cr(VI) removal from 50 mL of a [Cr(VI)](0) = 10 mg L-1 solution were: pH(0) = 3.6, t = 5 min, [GWS] = 6 g L-1, and I = 0.1 M. The corresponding removal efficiency was 95%. The kinetic of the reaction was also studied. Results indicate that using GWS is an affordable, efficient, and applicable method for aqueous Cr(VI) removal and thus a low-cost material for water treatment.