Migration and transformation pathways of chlorine and sulfur in producing pyrolytic biochar of a Zn/Cd-remediating plant amended with modified kaolin

Abstract

Pyrolysis of metal-rich post-harvest biomass from remediation sites to produce green, value-added products has gained interest. However, the presence of Cl and S can trigger the migration and transformation of metals during biochar production, limiting its clean utilization. This study not only explored the pyrolysis potential of Pfaffia glomerata (PG) biomass but also quantified the migration and transformation of Zn, Cd, and their mineral phases and interactions with Cl and S in response to the additive use of kaolin versus its modified form (MK) through thermal pre-activation and acid impregnation. The MK exhibited a high relative enrichment factor (R_ef) for Zn between 350–750 °C. Kaolin enhanced the immobilization of Cl and S in the solid biochar phase and promoted their removal from the gas phase. In particular, the additives promoted the SO₂ and HCl emissions between 200–400 °C, possibly due to the competitive adsorption of Si and Al with K and Ca. Sulfur-binding to Ca indirectly influenced the fixation and stabilization of Zn and Cd. In contrast, Cl directly promoted the release of Zn and Cd at the high temperatures. The addition of MK significantly improved kaolin’s ability to adsorb and immobilize Zn and Cd within the biochar, thus mitigating their potential environmental risks. For predicting jointly optimal biochar yield and quality, a multi-response artificial neural network (ANN) outperformed single-response generalized regression and boosted ANN and identified PG or PG mixed with 10 % kaolin (PK91) as the preferred feedstock type, along with specific amounts of Si, S, Al, K, Ca, and Cl.