Carbonization and ball milling on the enhancement of Pb(II) adsorption by wheat straw: Competitive effects of ion exchange and precipitation.

Straw biomass is a promising adsorbent for the removal of heavy metals. To improve its Pb(II) adsorption capacity and elucidate competition of adsorption mechanisms (e.g., ion exchange and precipitation), the Pb(II) adsorption mechanisms for wheat straw (WS-CK), wheat straw-biochar (WS-BC), and ball-milled wheat straw-biochar (WS-BC + BM) samples were investigated in detail by EDX, XRD, and FTIR. The results implied that the Pb(II) adsorption capacities at an adsorbent dosage of 0.2 g/L onto WS-CK, WS-BC, and WS-BC + BM were 46.33, 119.55, and 134.68 mg/g, respectively. This indicates that carbonization and ball milling are efficient techniques for improving the adsorption capacity of Pb(II) onto wheat straw, as WS-BC and WS-BC + BM exhibited adsorption capacities comparable to other commonly used bioadsorbents. Carbonization contributed significantly to precipitation (e.g., PbCO3 and Pb3(CO3)2(OH)2). Furthermore, competition existed between ion exchange and precipitation during the Pb(II) adsorption process. With relative lower adsorbent dosages, carbonization and ball milling enhanced ion exchange capacity.