Comparative Screening of Residual Biomass Hydrochars as Low-Dose Amendments for Short-Term Metal Retention in an Alkaline Soil Slurry

Abstract

Hydrochars (HCs) were produced from pine needles (urban waste), brewer’s spent grain, and grape pomace via hydrothermal carbonization (HTC) at 230 °C, and were comparatively assessed as unmodified, low-dose amendments (1 wt%) in an alkaline soil slurry. Fixed-contact-time (4 h) batch tests were conducted using a Taguchi L9 array to screen the effects of feedstock type, HTC residence time, initial metal concentration, and test temperature on the aqueous-phase removal of for Zn, Pb, and Cd at this fixed endpoint. In the ternary test containing 20 mg/L of each metal, the soil-only control removed 94% Zn, 99% Pb, and 96% Cd, whereas the soil amended with 1 wt% pine-needle HC removed 98% Zn and 99% Pb, while Cd in the recovered liquid was below the instrumental detection limit. Operational distribution coefficients (Kd) and separation factors (SF) calculated at the 4 h endpoint indicated preferential Cd partitioning within the amended soil system. These findings support further evaluation of minimally processed, low-dose hydrochars for metal-contaminated soils, while providing a potential valorization route for residual biomass within a circular economy framework.