Settling of copper-rich suspended particles from acid drainage neutralization as a function of chemical composition and particle size distribution

Abstract

The attenuation of total and dissolved metals from acid drainage (AD) results from the interaction of physical andchemical processes occurring during and after AD neutralization in impacted rivers. Chemical removal occurswhen dissolved metals are transferred into fine particles or flocs, while physical removal occurs when metal-richsuspended particles are deposited from the water column onto the riverbed. Most works studying metal atten-uation in rivers focus on characterizing chemical and physical removal processes separately, yet these processesoften interact and take place concurrently. The fate of copper (Cu) in particle suspensions formed from partialneutralization of AD was studied using a settling column coupled to a device that measured particles size dis-tribution in situ, with minimal floc disruption. Chemical composition, particles size distribution, and total sus-pended solids (TSS) were measured as particles settled. The physical removal of freshly formed particles wasenhanced by aluminum (Al), promoting the formation of larger particles and higher rates of TSS removal.However, Fe promoted higher partition of Cu onto particles, controlling its chemical removal. While the rate ofCu association to particles increased over time, TSS settled out from solution within the initial hour, regardless ofthe chemical composition within the range of tested conditions. Therefore, different remediation strategies maybe applied depending on the goal for removal (e.g., removal of TSS, removal of Cu) and water composition. Thesestrategies must consider chemical conditions (i.e., pH and concentrations of Al and Fe), settling times, and mixingconditions during particles formation for an optimal removal.