Removal of Cr(VI) from aqueous solutions by modified walnut shells

► The aim is to develop a new adsorbent by modifying walnut shells (WNS). ► The number of functional groups on the WNS increased with CA modification. ► Modified WNS is an efficient adsorbent for eliminating Cr(VI) from aqueous solutions. ► The capacity of this adsorbent is high for the removal of Cr(VI) ion. ► The sorption capacities of CA-WNS and WNS were 0.596 and 0.154 mmol/g, respectively.