Influence of Charring Methods on Surface Characteristics and Sorption Properties of Rice Straw Derived Biochars

Rice straw derived biochars were prepared by two different charring methods, in order to investigate effect of the preparation procedures on the sorption efficacy toward heavy metals from aqueous solutions.

The preparations of the biochars were under limited oxygen conditions, including one step heating treatment at 450°C/60 min. (Char 450) and two steps heating treatment upto 700°C/60 min. (Char 700). Influences of the charring methods on the surface characteristics of the obtained biochars were examined via scanning electron microscopy coupled with Energy Dispersive X ray Spectrometry (SEM-EDAX) and Fourier transform infrared spectroscopy (FTIR) analyses. The sorptive properties of Char 450 and Char 700 were evaluated according to their efficacy towards the removal of Cu2+ ions from the aqueous solutions. The sorption isotherms of Cu2+ onto the prepared biochars were analyzed by Freundlich and Langmuir models. Kinetic studies were also carried out.

Enhancements of the porous structure and some elemental content were depicted in Char 700 compared with Char 450, indicating the remarkable effect of the charring methods on surface properties of biochars. Both sorbents were able to remove all the Cu2+ ions at low initial sorbate concentrations and within 15 min. of contacting time. Further increase of the sorbate initial concentrations, Char 700 exhibited higher sorption efficiency than Char 450, achieving 66 and 55 % removal percentages, respectively.

At 2.0x10-3 M initial Cu2+ concentration, two hours of contact time were enough for the sorption of 69 % of the total ions. Langmuir isotherm equation and pseudo second order kinetic models successfully describes the sorption of Cu2+ onto the studied sorbents. Chemisorptions processes were assumed to be the domain sorption mechanisms.

The study indicated the potential use of the rice straw derived biochar as low cost sorbent in the field of wastewater treatment, taken in consideration the role of preparation conditions.