Enhanced Degradation of Trichloroethylene with Hydroxylamine Hydrochloride Addition in Citric-Acid Chelated Fe²⁺-Catalyzed Percarbonate System

The impact of hydroxylamine hydrochloride (HAH) addition on the enhancement of trichloroethylene (TCE) degradation in the citric acid (CA)-chelated Fe²⁺-catalyzed sodium percarbonate (SPC) system was investigated. The effects of HAH dosage and solution martirx (Cl^-, HCO₃^- and hurmic acid) on TCE removal were examined and the results indicated that HAH promoted TCE degradation significantly by transferring Fe³⁺ to Fe²⁺, therefore maintaining a sustainable activation of Fe²⁺. Under the optimum molar ratio of n_SPC:n_Fe²⁺:n_TCE=5:3:1 at the initial concentration of TCE 0.15 mol/L and CA 0.50 mmol/L, the final TCE removal efficiency was 99.6%. HCO₃^- had significant inhibitory effect on the degradation of TCE, whereas the effects of Cl^- and humic acid on the degradation of TCE could be ignored. In addition, TCE removal inhibition rate of HCO₃^- were 59.9%, 15.5% and 14.9% respectively when HCO₃^- concentration was all 1.0 mmol/L in SPC/Fe²⁺/TCE, SPC/Fe²⁺/CA/TCE and HAH/SPC/Fe²⁺/CA/TCE systems. The results indicated that HAH alleviated HCO₃^- inhibitory effect on the degradation of TCE. Scavengers tests were also conducted to understand the roles of ·OH and O₂^-· on TCE degradation. The results showed that ·OH and O₂^-· contributed 79.2% and 21.9% to TCE removal, respectively. The consistency of Cl^- concentration both experimentally and theoretically suggested the complete dechlorination of TCE in HAH/SPC/Fe²⁺/CA/TCE system. The intermediate products were formic acid, NO₂^- and NO₃^-. The concentration of formic acid increased to 5.8 mg/L within 60 min and then decreased to 1.4 mg/L within 120 min, meaning that the addition of HAH favors further degradation of formic acid and therefore complete mineralization of TCE. The overall results strongly confirmed the benefits of HAH amendment to TCE degradation in the SPC/Fe²⁺/CA system which provided a technical basis for practical applications in TCE contaminated groundwater remediation.