Unravelling the Effect of Sediment Properties on As(V) and As(III) Adsorption/Desorption Processes: Implications for Groundwater Geochemistry

Arsenic (As) mobility in aquifer systems is mainly governed by its adsorption and desorption behaviour at the sediment-water interface, directly influencing its environmental availability and risks to water quality. This study explores the adsorption-desorption behaviour of inorganic As species through batch experiments on environmental sediments collected from three representative depths, selected to reflect local contrasting geochemical, mineralogical, and granulometric characteristics of the Como basin aquifer (Northern Italy). This setting was selected as a case study owing to its notable gradient in As concentration in groundwater: the shallow aquifers host concentrations typically below 10 µg/L, while the deep aquifer reaches concentrations of about 250 µg/L. Statistical analyses (ANOVA and simple linear regression) identified Mn- and Al-(hydr)oxide content, grain size, and mineralogy as strong predictors of As(V) retention, whereas As(III) showed no significant correlation with individual sediment properties within the tested conditions. Shallow, Mn- and Al-rich sediments exhibited higher adsorption capacity and corresponded to lower dissolved As in groundwater, while deeper, finer-grained sediments with lower oxide content coincided with elevated groundwater As concentrations. Desorption experiments indicated that As(III) dominated the released fraction, reflecting its greater mobility under variable pH and redox aquifer conditions. These results provide mechanistic insight into sediment-water interactions controlling As distribution in multilayer aquifers, supporting improved risk assessment and management of As in complex groundwater systems.