Monovalent−Divalent Element Quotient Drives Lead(Ii) Removal in Aqueous Solutions by Nitric Acid-Modified Biochar

We evaluated the effects of elements that dissolved from nitric acid-modified biochars into aqueous solutions on lead(II) removal and the associated mechanisms for biochars derived from four biomasses at three production temperatures. Monovalent-divalent element quotient was calculated as the sum of monovalent element concentrations divided by that of divalent element concentrations to study their relative effects on lead(II) removal. Lead(II) removal reached 54% and increased with increasing monovalent-divalent element quotient due to increasing chemical activity. Compared to the pristine biochars, activation with nitric acid increased lead(II) adsorption rate, although the effect on the maximum adsorption capacity was mainly biomass-depended, increased for sawdust biochars but decreased for canola straw, manure pellet and wheat straw biochars. Regardless of biomass type and production temperature, lead(II) was mainly bounded as Pb-O and PbCO3 on the modified biochars through surface complexation and cation-π interaction, and the adsorption was best explained by Freundlich and pseudo-second-order models. However, precipitation and cation exchange were negligible in the adsorption process because minerals and elements were significantly dissolved from the biochars during the activation. Our results suggest that nitric acid activation is effective to increase lead(II) adsorption for biochars produced from mineral−depleted biomasses, such as sawdust