Removal of Pb(II) from contaminated water using low-temperature pyrolyzed agricultural and forest waste biochars: a comparative study

This paper investigates the use of biochars prepared from abundant agricultural and forestry wastes in the removal of toxic Pb(II) ions from aqueous solutions. The biochars were prepared by the pyroliza- tion at 400°C of saw dust, rice straw and rice husk. The efficacy of biochar in Pb(II) removal depends on its grain size, the dose of the biochar, pH and the initial concentration of the metal ion solution. The optimum adsorption was found to be within 1 h at a pH range of 5.0–7.0. Three isotherm models were employed to describe the Pb(II) adsorption on to biochar surfaces. Among these, the Langmuir isotherm model was a better fit for saw dust and rice straw biochar gave the highest regression coefficients of 0.97 and 0.95, respectively. But rice husk obeys the Langmuir model only at low concentrations with a regression coefficients of 0.956. The separation factor (RL) for saw dust and rice straw biochars were 0.234 and 0.128; this indicates a favourable sorption isotherm for Pb(II). The results of this study showed that 90%–94% removal of Pb(II) ion from aqueous solution could be attained by using saw dust biochar. The Freundlich model fitted well with the experiment data for all three sources of biochars. Only rice husk biochar fitted the Temkin isotherm model. The adsorption capacity of the biochars was in the order of saw dust>rice straw>rice husk. The FTIR spectra of three biomass samples indicated that Pb(II) metal can be adsorbed by the biochars specifically through the formation of surface complexes between Pb(II) and the functional groups on biochars.