<![CDATA[Nickel mining in high?rainfall tropical regions produces metal?rich runoff that can substantially alter biogeochemical processes at the water–sediment interface. However, the mechanisms governing nickel partitioning into dissolved, particulate, and sedimentary geochemical fractions in tropical river systems remain insufficiently resolved. This study investigates the fractionation and mobility of Ni, Fe, Mn, Cr, Cu, and Zn in a mining?impacted river in North Maluku, Indonesia, by integrating dissolved–particulate metal separation in water with five?step sequential extraction in sediments. Most metals in sediments were predominantly bound to the residual (RES) fraction, reflecting strong lithogenic control and incorporation into mineral lattices. In contrast, the exchangeable (EXC) fraction—representing the most mobile geochemical phase—followed the order Ni > Zn > Cu > Cr > Mn > Fe, indicating higher potential mobility for nickel relative to other metals. River water showed elevated proportions of dissolved Ni compared to its particulate form, suggesting substantial anthropogenic contributions from mining?derived geochemical runoff. The contrasting dominance of residual?bound metals in sediments and dissolved metals in water highlights a dual?source system in which sedimentary metals are primarily geogenic, whereas dissolved metals are strongly influenced by mining effluents. These findings advance understanding of nickel behavior at the water–sediment interface in tropical rivers and provide a mechanistic basis for predicting metal mobility and ecological risks in mining?impacted environments.]]>
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