Temperature-dependent transformation and solubility of trace metals in palm kernel shell ash

Biomass ash contains both plant-essential nutrients and potentially hazardous trace elements, and its safe utilization requires an understanding of how metal mobility and speciation evolve during thermal processing. This study investigated the temperature-dependent solubility and chemical speciation of trace elements in palm kernel shell (PKS) ash combusted at 100–1100 °C for 3 h. Changes in metal mobility were evaluated using total, water-extractable, and sequestered fractions, together with microscopic observations and X-ray absorption fine structure (XAFS) analysis. Untreated PKS contained Cr and Ni primarily as metallic contaminants derived from stainless steel abrasion during mechanical crushing. With increasing combustion temperature, Cr exhibited pronounced redox-dependent behavior: the highly soluble and toxic Cr(VI) increased between 500 and 900 °C, corresponding to a sharp rise in water-extractable Cr, but was substantially reduced at ≥1000 °C as Cr was stabilized in less soluble Cr(III)-dominated phases. In contrast, Ni remained largely water-insoluble across all temperatures. HCl-extractable Ni peaked at 500 °C, decreased at intermediate temperatures, and partially reappeared in a more soluble oxidized form at 1100 °C. Lead and cadmium showed negligible water solubility throughout the temperature range. Overall, combustion near 1000 °C represents an optimal processing window that minimizes the mobility of Cr and Ni while maintaining effective sequestration of Pb, Cd, and Ti, providing critical insights for the thermal treatment and safe reuse of biomass ash in agricultural applications. • Temperature controls trace metal solubility and speciation in PKS ash. • Cr shows redox-driven mobility, peaking as soluble Cr(VI) at 500–900 °C. • Ni remains water-insoluble, but becomes more reactive at 1100 °C. • Pb, Cd, and Ti are strongly sequestered across all combustion temperatures. • 1000 °C combustion minimizes metal mobility for safer ash reuse as fertilizer.