LAUSR

Abstract The Al constituents of soils and sediments play an important role in the bioavailability and fate of trace heavy metals. Pure Al precipitates rarely exist; instead, Al3+ is commonly coprecipitated with organics to produce organo–Al hydroxide composites. Herein, we replicate the Al hydroxide organo−mineral composite formation in soils and sediments by synthesizing composites using humic acid (HA) during coprecipitation with Al hydroxide (Al(OH)3). The mineralogy and morphology results indicate the formation of amorphous and nanosized pure Al(OH)3 and Al(OH)3 HA composites, while HA reduces the SSA and aggregation of the pure mineral. The sorption of Cu is enhanced on the Al(OH)3 HA composite over pure Al(OH)3 at pH   Al(OH)3. The Cu uptake by the Al(OH)3 HA composite is an energy-favorable process accomplished by an increase in entropy, signifying the formation of Cu-ligand inner-sphere complexes. Nano secondary ion mass spectrometry (NanoSIMS) results demonstrate that Cu is mainly sorbed on the HA fraction of the Al(OH)3–HA composite. This study therefore has important implications for predicting the cycling of trace heavy metals in Al-rich geologic environments, and sheds light on the combined use of ITC and NanoSIMS to uncover the distribution and speciation of trace elements in complex mineral–organic composites.