Assessments of Heavy Metals Accumulation, Bioavailability, Mobility, and Toxicity in Serpentine Soils

Accumulation of heavy metals is a concerning issue due to their known persistence in the ecosystem, and there are standard limits established for their maximum allowable concentrations in soils. However, heavy metal accumulation coming from serpentinite soils often exceeds the regulatory values, and there is a lack of knowledge regarding their bioavailability, mobility, and toxicity in the environment. This research applied novel selective sequential extraction and leaching procedures to assess the gaps in knowledge regarding heavy metals accumulation on serpentinite topsoil derived from a few states in Peninsular Malaysia. Based on the total digestion method, the concentration of all studied heavy metals except Mn exceeded the site screening levels issued by the Department of Environment, Malaysia (DOEM). The Geo-accumulation Index categorized Cr, Cd, Ni, and Co as extreme contamination and Cu, Pb, Zn, and Mn as unpolluted to moderate contamination. From the extraction results, Cd was found bounded 100% to a residual fraction. Meanwhile, Ni, Co, and Cr were mostly (≥92%) found to be bound to a residual fraction, with the remaining percentages distributed within non-bioavailable fractions (crystalline Fe oxides, poorly crystalline Fe oxides, and Mn oxides). Nevertheless, Cu, Pb, Zn, and Mn contaminants showed an increase (1–9%) in bioavailability and mobility fractions (soluble–exchangeable, surface-adsorbed, and organic matter) which pose a threat to the environment. The toxicity of the heavy metals greatly surpassed the DOEM standards; however, it was still below the global USEPA toxicity control. This research concluded that, even though the toxicity level of the topsoils had not exceeded the global toxicity limit, the accumulation of heavy metals in the serpentinite soils needs to be addressed due to its high concentration and its being potentially bioavailable and mobile in the environment.