Influence of chemical enhancer injection timings on electrokinetic geosynthetics-assisted vacuum preloading combined with electroosmosis for dredged sediments dewatering and heavy metal removal

Abstract

Effective dewatering and heavy metal removal of dredged sediments using the electrokinetic geosynthetics-assisted vacuum preloading combined with electroosmosis (VPE) technique can convert dredged sediments into valuable resources for construction use. Chemical enhancers are commonly employed to enhance VPE performance, but their optimal injection timing remains largely unclear. This study investigated the influence of ethylenediaminetetraacetic acid (EDTA) and citric acid (CA) as well as their injection timings on the VPE efficacy for dredged sediment improvement. It compared drainage volume and drainage rate, moisture content, electric current and effective voltage, Cu concentration and speciation, and energy consumption under early- (I_t = I₀), mid- (I_t = 70 %I₀), and late-stage (I_t = 40 %I₀) injections. The results showed that CA generally outperformed EDTA, achieving approximately 200 mL greater final drainage volumes and averagely reducing Cu concentrations from ∼1640 mg/kg to ∼745 mg/kg (EDTA was ∼755 mg/kg). Late-stage CA injection optimized drainage efficiency and moisture content uniformity. It reduced the moisture content gradients between the cathode and the anode to ∼4 % (it was ∼12 % for the injection of I_t = I₀). The mid-stage injection of CA may achieve a balance between Cu removal and energy efficiency. EDTA, while less energy-intensive, retained higher weak-acid extractable Cu (∼15 %), posing leaching risks.