Adsorbent hydrogel electrode of kappa-carrageenan-activated carbon double crosslinked with glutaraldehyde/FeCl₃ for electrokinetic soil remediation

Adsorbent electrodes combine the advantages of adsorbents and the outstanding conductivity of conventional electrodes. The main challenges to using hydrogel electrodes in the electrokinetic (EK) process are due to their compromised mechanical strength, chemical stability and degradation under the harsh soil environment. This study investigated adsorbent kappa-carrageenan (kC) hydrogel electrodes with excellent mechanical strength, adsorption capacity, and chemical stability. The adsorbent electrodes were prepared by mixing kC with 0.3% activated carbon (AC) and were double crosslinked with glutaraldehyde (GD) and FeCl₃ to enhance their physicochemical and adsorption characteristics. Kaolinite and real soil were spiked with Zn²⁺, and the EK process was conducted with graphite, as a baseline test, and adsorbent hydrogel electrodes for comparison purposes. The EK with graphite electrodes achieved 3.37% Zn²⁺ removal from the kaolinite soil, compared to up to 97.71% Zn²⁺ removal with hydrogel electrodes due to the suppression of the alkaline front generated at the cathode via acid–base interactions and metal-ion coordination with the hydrogel electrodes. In the real soil, Zn²⁺ removal was 62.32% and 68.22% in the EK with the hydrogel electrodes for 14 and 21 days. The lower Zn²⁺ removal in the real soil may be explained by the slow propagation of the acid front through the soil matrix and the interaction between Zn²⁺ and the soil's organic matter. Remarkably, the kC-based hydrogel adsorbent electrodes were still intact even after 21 days. This finding highlights the promising potential of bio-hydrogels for decarbonizing soil remediation processes.