Non-monotonic effects of soil particle size on UV-assisted electrokinetic remediation of Cr(VI) contaminated soils

Purpose Understanding how soil characteristics influence remediation is critical for optimizing treatment strategies. This study investigates how soil particle size influences the efficiency of UV-assisted electrokinetic remediation (UV-EKR) for Cr(VI)-contaminated soils. The primary objectives are to evaluate the effects of particle size on electrokinetic transport and photochemical reactivity. 
Materials and methods Four soil fractions (< 150 m, 150-335 mu m, 335-1000 mu m, 1000-2000 mu m) were prepared by dry sieving. UV-EKR experiments were conducted to assess electrokinetic behavior, photoreactive interface dynamics, and Cr(VI) removal under controlled conditions. Electroosmotic flow, electrolyte, soil resistivity and pH, and Cr(VI) concentration profiles were monitored, and multivariate analysis was used to relate spatial heterogeneity to electrokinetic reaction gradients. 
Results and discussion Fine-grained soil (SG4, < 150 m) exhibited enhanced photochemical precipitation of Fe/Cr (oxy) hydroxides in the anode region, leading to pore blockage, reduced electroosmotic flow, and increased resistivity. While finer soils (UV-EKR3-4) exhibited higher electrokinetic responsiveness, facilitating Cr(VI) removal in cathode-proximal zones (S4-S5) via alkaline desorption and elevated current density, coarse soils (SG1) maintained more stable transport and resisted pore clogging, favoring contaminant removal in acidic, dissolution-dominated anodic regions (S1-S2). Despite these contrasting behaviors, overall removal efficiencies remained comparable across soils (36.98-41.90%). UV photolysis of Fe(III)-citrate effectively generated Fe(II) to drive Cr(VI) reduction, yet long-term performance was limited by soil restructuring and mineral re-precipitation. Multivariate regression confirmed that spatial position was the primary determinant of Cr(VI) removal (beta= 0.044, p = 0.001), whereas particle size was not significant (p = 0.945). Acidic fronts at the anode dissolved immobile Cr, while alkaline fronts at the cathode enhanced mobility. 
Conclusions Particle size non-monotonically influenced UV-EKR efficiency through pore clogging and dissolution pathway differences, yet spatial heterogeneity was the dominant determinant of Cr(VI) removal. Fine soils enhanced reactivity but faced clogging, while coarse soils ensured stable transport. Effective application requires particle-size-specific optimization to balance reactivity, stability, and ecological safety.