酸碱及氧化剂对聚偏氟乙烯超滤膜老化的影响

Effect of different chemical cleaning agents on the ageing of PVDF ultrafiltration membrane

摘要:

为研究水处理中超滤膜的老化规律, 利用低浓度的氢氧化钠(NaOH)、碳酸钠(Na₂CO₃)、次氯酸钠(NaClO)、盐酸(HCl)和柠檬酸(H₃Cit)老化聚偏氟乙烯超滤膜.通过纯水通量、TOC截留率、扫描电镜图像、拉伸性能、傅里叶红外光谱等方法表征超滤膜老化的特性, 探索生产中使用的酸碱及氧化剂对PVDF超滤膜结构和性能的影响.结果表明, 酸、碱及氧化剂的使用均会导致PVDF膜的结构及性能退化, 特别是NaOH和NaClO, 其影响更为明显.经NaOH、Na₂CO₃和NaClO老化75 d后, TOC截留率从36%分别降低至15%、27%和17%.NaOH老化15 d后断裂拉力和伸长率降低11%和20%.碱和氧化剂对膜的截留效果及机械性能的影响大于酸清洗剂.NaClO对膜表面亲水性的影响显著高于其他清洗剂.PVDF的脱氟化氢反应产生C＝O和C＝C键是超滤膜老化的直接原因之一.

Abstract:

To investigate the aging characteristics of the ultrafiltration membrane used in water treatment, the polyvinylidenefluoride (PVDF) ultrafiltration membrane was aged with sodium hydroxide (NaOH), sodium carbonate(Na₂CO₃), sodium hypochlorite (NaClO), hydrochloric acid (HCl) and citric acid (H₃Cit), respectively. The effect of low-concentration cleaning agents on the aging of the PVDF membrane was investigated, with the analyzing of the parameters of pure water flux, total organic carbon (TOC) rejection, tensile strength, field emission scanning electron microscopy (FESEM), contact angle and fourier transform infrared (FTIR). Experimental result shows that acid, alkali and oxidant, especially NaOH and NaClO, changed the structure and performance of the PVDF ultrafiltration membrane. After exposed to NaOH, Na₂CO₃ and NaClO for 75 days, the total organic carbon (TOC) rejection efficiencies decrease from 36% to 15%, 27% and 17%, respectively. Compared to the virgin membrane, the tensile strength and ultimate elongation of the membrane which aged 15 days with NaOH declines by 11% and 20%, respectively. The negative effect of alkali and oxidant on mechanical performance and TOC rejection was higher than that of HCl and H₃Cit, and the decreasing of hydrophilicity of the membrane exposed to NaClO is more significant than that of the other cleaning agents. The dehydrofluorination, which can generate of C＝C and C＝O bond, is one of the most important reasons that caused the ageing of PVDF membrane.