Sediment metals adhering to biochar enhanced phosphorus adsorption in sediment capping

Abstract

Metal ions in sediment are inherent Ca and Fe sources for biochar modification. In this work, the effect of Ca2+ and Fe2+ released from sediment on biochar for phosphorus adsorption was evaluated. Results showed that raw peanut shell biochar (PSB) was poor in phosphorus adsorption (0.48 mg/g); sediment-triggered biochar (S-PSB) exhibited a P adsorption capacity of 1.32 mg/g in capping reactor and maximum adsorption capacity of 10.72 mg/g in the Langmuir model. Sediment released Ca2+ of 2.2–4.1 mg/L and Fe2+/Fe3+ of 0.2–9.0 mg/L. The metals loaded onto the biochar surface in the forms of Ca-O and Fe-O, with Ca and Fe content of 1.47 and 0.29%, respectively. Sediment metals made point of zero charge (pHpzc) of biochar shifted from 5.39 to 6.46. The mechanisms of enhanced P adsorption by S-PSB were surface complexation of CaHPO4 followed by precipitation of Ca3(PO4)2 and Ca5(PO4)3(OH). Sediment metals induced the modification of biochar and improvement of P adsorption, which was feasible to overcome the shortcomings of biochar on phosphorus control in sediment capping. HIGHLIGHTS Biochar was triggered by sediment Ca2+ and Fe2+.; Metals loaded on biochar surface as Ca-O and Fe-O.; q of S-PSB was 2.5 times higher than PSB.; Metal-loaded biochar was a seed for Ca-P complexation and precipitation.;

Keywords

• biochar
• ca2+
• endogenous phosphorus
• metal ions
• sediment capping