Arsenic(III) removal from aqueous solution by raw and zinc-loaded pine cone biochar: equilibrium, kinetics, and thermodynamics studies

Abstract

The potential of raw pine cone (PC) biochar and its modified form (Zn-loaded biochar) was evaluated for the removal of trivalent arsenic [As(III)] from the aqueous solution. The influence of treatment of PC biochar with Zn(NO3)2 on the physico-chemical properties of biochar was examined using elemental analyzer, surface area analyzer, thermo-gravimetric analyzer, Fourier transform infrared spectroscopy, and scanning electron microscopy. Experimental results revealed that the removal of As(III) was almost consistent (66.08 ± 3.94 and 87.62 ± 3.88 % on raw and Zn-loaded biochar, respectively) over an acidic pH range of 2–4 as a consequence of high affinity between the positively charged biochar surface and the predominant arsenic species (H3AsO3 and H2AsO4 −), followed by a decrease with increase in pH in the range of 4–12. Langmuir isotherm model was well fitted to the experimental equilibrium data (R 2 = 0.93) rendering the maximum adsorption capacity of 5.7 and 7.0 μg g−1 on raw and Zn-loaded PC biochar, respectively. The adsorption of As(III) was well represented by pseudo-second order with reaction rate constant (k 2) of 0.040 and 0.282 mg g−1 min on raw and zinc-loaded PC biochar, respectively, and was mainly controlled by boundary layer diffusion followed by some extent of intra-particle diffusion. The adsorption process was spontaneous with Gibb’s free energy (ΔG°, −4.42 ± 0.72 and −11.86 ± 1.78 kJ mol−1) and exothermic in nature with enthalpy (ΔH°, 13.25 and 31.10 kJ mol−1) and entropy (ΔS°, 0.058 and 0.141 kJ mol−1 K−1) for raw and zinc-loaded adsorbent, respectively.