Abatement of chromium by adsorption on nanocrystalline zirconia using response surface methodology Deepak Gusain a , Faizal Bux b , Yogesh Chandra Sharma a, ⁎ a Department of Chemistry, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi 221 005, India b Institute for Water and Wastewater Technology, Durban University of Technology, PO Box 1334, Durban 4000, South Africa abstract article info Article history: Received 19 March 2014 Received in revised form 26 April 2014 Accepted 27 April 2014 Available online 9 May 2014 Keywords: Nanocrystalline zirconia Chromium Removal Response surface methodology Application of nanocrystalline zirconia was investigated for the removal of chromium from aqueous solutions. The nanozirconia was synthesized by ‘precipitation method’. The parameters namely initial concentration (5–65 ppm), pH (0.5 to 10.5), adsorbent dose (2 to 10 g/l) and temperature (298 K to 318 K) were optimized employing central composite design of response surface methodology. The removal of Cr was most affected by pH, followed by an ini- tial concentration and adsorbent dose. Maximum removal (%) was achieved at an initial concentration of 20 ppm, pH = 3, adsorbent dose = 4 g/l and temperature 313 K. The experimental data were best fitted in Langmuir's iso- therm equation and the removal followed pseudo second order kinetics. The mechanism of removal was explained by boundary layer diffusion via intraparticle diffusion and was further confirmed by Boyd plot. Thermodynamic pa- rameters revealed that the removal process was spontaneous, endothermic and physiosorptive in nature. Adsor- bent was regenerated with hydroxides (0.1 N NaOH, KOH and NH 4 OH) for further reuse. © 2014 Elsevier B.V. All rights reserved. 1. Introduction All living beings including humans require water to survive except in certain ecosystems like hydrothermal vents. Water employed in various sectors is concisely categorized into three categories i.e. industry, agri- culture and domestic. The ever increasing population has resulted in an immense stress on water quantity and quality. The pressure on water resources is further added by increasing population, industrializa- tion and modern agricultural practices. Heavy metal pollution is one of the problems retro-grading the aqueous ecosystems. Chromium is one of the metals with applications in various industries such as, electroplating, textile, stainless steel, leather, and paint industries. Higher intake of chromium in human body results in weakening of im- mune system, DNA strand breaks, alteration in cellular signaling path- way, ulcers, allergic dermatitis and ultimate death in many cases [1–3]. Because of well documented adverse health effects of chromium and its widespread applications, it is mandatory to remove it from industrial effluents prior to discharge. Chemical precipitation, coagulation and flocculation, electro-coagulation, ion exchange, membrane separation, nano-filtration, solvent extraction, reverse osmosis [4–10], and adsorp- tion [11–14] are some of the techniques widely reported for the treat- ment of chromium laden aqueous solutions and waters. Each of these techniques has merits but they suffer from certain drawbacks like high operational cost, along with regeneration and need of trained per- sonnel. But adsorption has advantages over other technique due to ease in operation, regeneration, high efficiency, low energy input and remov- al of pollutants even at trace concentrations. Various adsorbents like ac- tivated carbon, carbon nanotubes, lignin, chitosan, clay, fly ash, bacteria, fungi, and nanostructured oxides such as iron oxide and aluminum oxide [12–23] have been reported for the removal of Cr from aqueous solutions. Present study addresses removal of chromium by adsorption on nanocrystalline zirconia. Zirconia has excellent chemical inertness and is known to be biocompatible with system [24]. Removal efficiency of the synthesized adsorbent, nanozirconia for removal of Cr was exam- ined and reported. Response surface methodology was used for optimi- zation of experimental parameters and thermodynamic studies for the removal of Cr were also carried out and reported. 2. Experimental 2.1. Materials and analytical instruments Potassium dichromate (K 2 Cr 2 O 7 ) and ammonium hydroxide (NH 4 OH) were procured from Merck, Mumbai, India. Zirconium oxychloride octahydrate (ZrOCl 2 ·8H 2 O) was obtained from Himedia, India. Tubular Furnace (IKON, India), Analytical balance (VIBRA), pH meter (IKON, India), X-ray diffractometer (MINIFLEX II, Desktop XRD, RIGAKU), DTA/TGA (Labsys™ TG–DTA 16, SETARAM Instrumentation), Scanning electron microscope (Quanta 200 f. FEI), Transmission electron microscope (TECNAI G2, FEI), water bath shaker (Narang scientific), and atomic absorption spectrophotometer (Szhimadzu AA 7000) were used in the present studies. Journal of Molecular Liquids 197 (2014) 131–141 ⁎ Corresponding author. Tel.: +91 542 6702865. E-mail address: ysharma.apc@iitbhu.ac.in (Y.C. Sharma). http://dx.doi.org/10.1016/j.molliq.2014.04.026 0167-7322/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq