Eco-Friendly Zinc Extraction from Sandy Soils via Electrokinetics Assisted by Tartaric Acid and Agricultural Waste

The contamination of agricultural sandy soils by zinc (Zn) presents not only a risk for the quality of crops but also a problem for the quality of groundwater. Common enhancing agents used in electrokinetic remediation (EKR) of heavy metals, such as EDTA, present persistence in the environment. This work evaluates an EKR technique that involves a biodegradable chelating agent, tartaric acid, and a barrier of untreated rice husk installed in front of the cathode. The experimental design includes a sandy soil (93.4% sand) artificially contaminated with 300 mg/kg Zn and treated for 5 days under the voltage gradient of 1 V/cm. Besides three experiments without enhancement, where the purging solutions were tap water adjusted to pH 3, 7 and 11, there was one experiment with tartaric acid at pH 3 as the anolyte. All experiments were done once. The removal of Zn decreased from 77.8% at pH 3 to 69.7% at pH 7 and 53.2% at pH 11 due to the higher desorption and electromigration of Zn in acidic conditions and the precipitation of Zn as Zn(OH)₂ in neutral and alkaline conditions. In all experiments, the residual amount of Zn decreased along the direction from the anode to the cathode. The removal of Zn in the tartaric acid-enhanced experiment was the highest (90.8%), which corresponds to 13 percentage points more than in the experiment without enhancement at pH 3, with the residual amount of Zn being 15–43 mg/kg in the soil column. This fact is due to the formation of soluble Zn-tartrate complexes and buffering action of tartaric acid that maintained soil pH within 4.2–5.7. The amount of Zn retained in the rice husk barrier was up to 38 mg/kg. Therefore, the EKR enhanced by biodegradable tartaric acid can be considered a potentially useful technique for the remediation of Zn-contaminated sandy soils. Replicate tests, the cost evaluation, the effect of the technique on soil quality and quantification of the barrier contribution are needed.