紫外/次氯酸钠和紫外/过碳酸钠工艺降解水杨酸的影响因素及降解机理

采用紫外/次氯酸钠(UV/NaClO)和紫外/过碳酸钠(UV/SPC)工艺降解水中水杨酸(SA)，且利用协同因子(R)作为评价指标，分别考察了氧化剂投加量、pH、阴离子(NO₃^-、HCO₃⁻)和腐殖酸(HA)等因素对SA去除的影响，结合TOC对比了2种工艺对SA的去除效果，通过鉴定中间降解产物探讨了SA可能的降解路径。结果表明：UV/NaClO和UV/SPC工艺中SA的去除均符合拟一级反应动力学，R与拟一级反应动力学常数(k_obs)变化趋势相似。当NaClO和SPC质量浓度分别为 3 mg·L⁻¹和12 mg·L⁻¹时，2种工艺中k_obs分别为0.173 2 min⁻¹和0.258 8 min⁻¹，而 R_UV/NaClO和R_UV/SPC分别为9.5和15.9。k_obs和R随氧化剂投加量的增加而升高，因过量的SPC消耗产生的羟基自由基(·OH)会导致k_obs降低。初始pH对SA去除有较大影响，酸性环境有利于UV/NaClO工艺去除SA，而UV/SPC工艺则在pH=7具有较好的SA去除效果。NO₃⁻与HCO₃⁻对UV/NaClO工艺去除SA有轻微的促进作用，而显著抑制UV/SPC工艺对SA的去除。HA对2种工艺中SA的去除均有抑制作用。相比UV/NaClO，UV/SPC工艺对TOC去除更为显著。通过分析SA的密度泛函理论(DFT)并结合主要的降解产物推测SA的降解机理主要为自由基的取代和氧化。

Ultraviolet/sodium hypochlorite and ultraviolet/sodium percarbonate processes for degradation of salicylic acid: Influencing factors and mechanism

The UV/NaClO and UV/SPC processes were used to degrade salicylic acid (SA) in water. Based on the synergy factor (R) as the evaluation index, the effects of oxidant dosage, pH, anions (NO3−, HCO3−) and humic acid (HA) on SA removal were investigated. Combined with TOC, the SA removal during UV/NaClO process and UV/SPC process was compared, the degradation path of SA and the possible intermediate products were also discussed. The results showed that SA removal in UV/NaClO and UV/SPC processes fitted well with the pseudo- first-order reaction kinetics model, and R and pseudo-first-order reaction kinetical constant (kobs) presented the similar variation trend. When the concentrations of NaClO and SPC were 3 mg·L−1 and 12 mg·L−1, respectively, the kobs values of the two processes were 0.1732 min−1 and 0.2588 min−1, respectively, the RUV/NaClO and RUV/SPC were 9.5 and 15.9, respectively. kobs and R increased with the increase of oxidant dosage, but the excessive SPC would consume the generated hydroxyl radicals (·OH), resulting in the decrease of kobs. The initial pH had a distinctly influence on both processes, the UV/NaClO process was more effective in an acidic environment than in an alkaline one, and the UV/SPC process demonstrated an excellent removal performance in a neutral environment (pH=7). NO3− and HCO3− could slightly promote SA degradation by the UV/NaClO process, but significantly inhibit SA degradation by the UV/SPC process. HA inhibited SA removal by both processes. UV/SPC process was superior to UV/NaClO process in TOC removal. Based on the density functional theory (DFT) analysis of SA and the main intermediate products, the main mechanism of SA degradation was the substitution and oxidation of radicals.