Removal of Zinc from Concentrated Galvanic Wastewater by Sodium Trithiocarbonate: Process Optimization and Toxicity Assessment

In the present research, the removal of zinc from concentrated galvanic wastewater (pH 3.1, conductivity 20.31 mS/cm, salinity, 10.16 g/L, Chemical Oxygen Demand (COD) 2900 mg O₂/L, Total Organic Carbon (TOC) 985 mg/L, zinc (Zn) 1534 mg/L and ethylenediaminetetraacetic acid (EDTA) 70 mg/L) by combination of lime (Ca(OH)₂) and sodium trithiocarbonate (Na₂CS₃) as precipitation agents is studied. Central Composite Design (CCD) and response surface methodology (RSM) were applied for modelling and optimizing the designed wastewater treatment process. Analysis of Variance (ANOVA) and the experimental verification of the model confirmed the consistency of the experimental and estimated data calculated from the model (R² = 0.9173, R²_adj. = 0.8622). The use of Ca(OH)₂ and Na₂CS₃ in the optimal condition calculated from the model (pH = 10.75 ± 0.10, V Na₂CS₃ dose 0.043 mL/L and time = 5 min) resulted in a decrease in the concentration of Zn in treated wastewater by 99.99%. Other physicochemical parameters of wastewater also improved. Simultaneously, the application of Ca(OH)₂ and Na₂CS₃ reduced the inhibition of activated sludge dehydrogenase from total inhibition (for raw wastewater) to −70% (for treated wastewater). Under the same conditions the phytotoxicity tests revealed that the seed germination index for the raw and treated wastewater increased from 10% to 50% and from 90% to 100% for white mustard (Sinapis alba) and garden cress (Lepidium sativum L.), respectively. The parameters of root and shoot growth showed a statistically significant improvement. Treated wastewater (1:10) showed a stimulating effect (shoot growth) compared to the control sample (GI = −116.7 and −57.9 for S. alba and L. sativum L., respectively). Thus, the use of Na₂CS₃ is a viable option for the treatment of concentrated galvanic wastewater containing zinc.