Potential of activated biochar for sequestration of chromium (VI) from aqueous solution: parameters optimised by RSM, Isotherm and kinetics study

ABSTRACT

Adsorption capacity of activated biochar derived from Eichhornia crassipes was investigated for the sequestration of Cr (VI) from aqueous solution. Biochar was prepared and activated with different agents viz., 85% phosphoric acid, 0.5% sulphuric acid, 1 M ferric chloride and 10% polyethylenimine. Adsorption parameters were optimised by using Box–Behnken model of Response Surface Methodology (RSM), where the P value lesser than 0.05 and higher F value explained the significance of the model. Concentration and time showed positive effect while pH, biosorbent dosage and biosorbent size represented negative effect on adsorption. The optimum conditions for adsorption of Cr (VI) by activated biochar obtained from the model were pH: 2, biosorbent dosage was 1%, biosorbent size 1 mm, solute concentration of 985 mg/L and equilibrium time of 35 hours. An increase in concentration of solute molecules increased the adsorption of Cr (VI), whereas percent removal was reduced. Equilibrium relationship between sorbent and sorbate was determined using the isotherms Langmuir, Freundlich and Temkin models. Isotherm study was supported by Langmuir where sorption occurs on monolayer. The rate of adsorption and its mechanisms elucidated through kinetic studies. The higher correlation coefficient of 0.9985 reveals the significance of the studied models. Kinetics study fits well with the pseudo-second-order kinetics whereas isotherm study was best described by Langmuir model with maximum adsorption capacity of 50.0 mg g⁻¹ under biochar activated with phosphoric acid (PA-WBC). Among the activation methods PA-WBC showed maximum chromium adsorption due to the influence of H⁺ ions present on the surfaces which highly favoured the sorption of anions. Hence, this activated biochar can be effectively utilised for the removal of Cr (VI) from industrial waste water, it was further suggested that water hyacinth has the additional advantage that it reduces or even eliminates the diverse impact of the weed on the environment.

KEYWORDS:

• Activated biochar
• biosorption
• response surface methodology
• Cr (VI) adsorption
• isotherm
• kinetics

Acknowledgments

The authors thank the Science and Engineering Research Board (YSS/2015/001432), Department of Science and Technology, New Delhi for financial assistance.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Science and Engineering Research Board, Department of Science & Technology, Government of India [YSS/2015/001432].