Impacts of carbonization temperature on the Pb(II) adsorption by wheat straw-derived biochar and related mechanism.

To determine the quantitative correlations between physicochemical characteristics and Pb(II) adsorption amounts of biochar fractions, we prepared wheat straw-derived biochar under various carbonization temperatures (300-900 °C). The different fractions of the wheat straw-derived biochar, water-soluble material (WM), acid-soluble material (AM), and organic material (OM), were acquired. The ash content, ultimate analysis, pH, ion strength (IS), cation exchange capacity (CEC), and acidic functional groups (AFG) were characterized. The Pb(II) adsorption amounts of different biochars and their fractions were determined. The results revealed that the proportions of biochar fractions (WM, AM, and OM) varied with various carbonization temperatures. The maximum Pb(II) adsorption amount of wheat straw-derived biochar (qTotal) was 157.95 ± 0.13 mg/g obtained at 800 °C, and the quantitative correlations between Pb(II) adsorption amount (q) and carbonization temperature (T) can be elaborated by qTotal = 170.72-336.62exp(-0.0035T) (R2 = 0.97), qWM = 106.18-390.10exp(-0.0046T) (R2 = 0.98), qAM = 496.16-477.74exp(-0.0001T) (R2 = 0.79), and qOM = 1.80 + 34.69exp(-0.0038T) (R2 = 0.85). For rate of contribution (RC) for Pb(II) adsorption, when T < 400 °C, the order was AM (60.72 ± 7.33%) > OM (23.41 ± 7.33%) > WM (15.87 ± 0.30%); however, when T ≥ 400 °C, the order was WM (52.31 ± 0.85% - 67.65 ± 2.99%) > AM (29.65 ± 0.46% - 35.77 ± 0.12%) > OM (2.30 ± 0.47% - 12.02 ± 2.43%). Moreover, qWM and qAM exhibited significant positive linear correlations with ash (qWM = 9.92Ash - 123.65, and qAM = 2.13Ash - 0.49), qTotal was predominantly affected by ash content (qTotal = 10.97 Ash - 95.49). The EDX, XRD, and FTIR analysis results further clarified that ion exchange and precipitation were the main adsorption mechanisms for Pb(II) adsorption by wheat straw-derived biochar.