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

Walnut shell (WNS) (Juglans regia) has been utilised as an adsorbent for the removal of Cr(VI) ions from aqueous solutions after treatment with citric acid. The modification reaction variables, such as citric acid level (5–10 g), reaction time (0–24 h), and temperature (110–130 °C) were studied in batch experiments. The rate of adsorption was studied under a variety of conditions, including initial Cr(VI) concentration (0.1–1.0 mM), amount of adsorbent (0.02–0.20 g), pH (2–9), temperature and contact time (10–240 min). Adsorption of Cr(VI) is in all cases pH-dependent showing a maximum at equilibrium pH values between 2.0 and 3.0 for citric acid modified walnut shell (CA-WNS). The applicability of the Langmuir, Freundlich and D–R adsorption isotherms has been tested for the equilibrium. Maximum adsorption capacities of CA-WNS and untreated WNS under experimental conditions were 0.596 and 0.154 mmol/g for Cr(VI) ions, respectively.