The Research On Forward Osmosis In Treating High-salinity Wastewater Containing Heavy Metal Zn²⁺and Ni²⁺

High-salinity wastewater containing heavy metal is one of sewages that are most difficult to be treated,and receives more and more attention with the acceleration of the industrialization process in China.Traditional processing methods tend to have small range of processing,serious secondary pollution,low recycling rate of heavy metal and so on.It is urgent to find a novel method to treat this kind of wastewater.FO can be an alternative method due to the high resistance to various pollutants,relatively low pollution tendency and high pollution reversibility.Therefore,in this study,the performance of forward osmosis（FO）in treating the high-salinity feed waters was investigated by using NaCl as the driving solute and high concentration of NaCl solution as high-salinity wastewater.The performance of FO in treating high-salinity wastewater containing heavy metals was studied by adding heavy metal ions Zn²⁺,Ni²⁺,mixed ions Zn²⁺and Ni²⁺to high concentration of NaCl solution.The effects of temperature,membrane orientation,salinity of feed solution（FS）on FO performance in treating high-salinity wastewater were first to be investigated,the performance index was water flux.The optimal operating conditions were determined through the study.Results showed that water flux decreases with the increase of FS salinity.When FS salinity is low,the effect of temperature and membrane orientation on water flux is greater:the water flux in the pressure retarded osmosis（PRO）mode is greater than that in the forward osmosis（FO）mode.And water flux obtained at 35℃ is greater than at 25℃ and slightly less than at 45℃.When FS salinity is high,the effect of temperature and membrane orientation on water flux is small.Considering the efficiency of treatment and energy consumption,35℃ was selected as the best treatment temperature.Then the water flux and the removal efficiency of Zn²⁺ were investigated at different temperature and membrane orientation by adding heavy metal ions Zn²⁺ to high concentration of NaCl solution.Results showed that the effect of Zn²⁺ on water flux is relatively small.The removal efficiency of Zn²⁺ was higher than 95%,and tend to be increased with higher FS salinity and lower initial concentration of Zn²⁺in FS.Furthermore,the removal efficiency of Zn²⁺ in FO mode was higher than that in PRO mode in low salinity FS,while the two modes had a similar removal efficiency of Zn²⁺ in high salinity FS.By the study of water flux and removal efficiency of Zn²⁺ in long-term mode,it can be seen that FO technology has a relatively optimal time for the treatment of high-salinity wastewater containing Zn²⁺ generally this time is 10-11 h.By adding heavy metal ions Ni²⁺ to high concentration of NaCl solution,the water flux and the removal efficiency of Ni²⁺ were investigated at different temperature and membrane orientation.The results showed that the addition of Ni²⁺lowered the water flux in low salinity FS,while slightly increased in high salinity FS.The removal efficiency of Ni²⁺ of the FO membrane was higher than 98%,and increased with the FS salinity.Furthermore,the removal efficiency of Ni²⁺ in FO mode was lower than that in PRO mode in low salinity FS while the situation was opposite in high salinity FS.But when the salinity of FS was higher,both models have a high rejection rate for Ni²⁺.The above results showed that FO applied in the treatment of high-salinity wastewater containing Ni²⁺ is feasible.In the feasibility study of using FO technology to treat high-salinity wastewater containing Zn²⁺ and Ni²⁺,the water flux and the removal efficiency of Zn²⁺ and Ni²⁺ were investigated by using NaCl solution containing Zn²⁺ and Ni²⁺ as FS.The driving effect was compared with that of the treatment of high-salinity wastewater（baseline）,high-salinity wastewater containing Zn²⁺ and high-salinity wastewater containing Ni²⁺.The water fluxes gained by four solutions were in sequence of:In low salinity FS,baseline>high-salinity wastewater containing Zn²⁺>high-salinity wastewater containing Zn²⁺ and Ni²⁺>high-salinity wastewater containing Ni²⁺.In high salinity FS,water flux gained by high-salinity wastewater containing Ni²⁺was slightly higher than baseline,while water fluxes gained by the rest three kinds of solutions were close.But in higher salinity FS,water fluxes gained by four different types of wastewater were relatively close.The removal efficiency of heavy metal ions in mixed solution higher than that in single solution.Studies on FO membrane scaling have shown that membrane fouling can be effectively removed by simple physical cleaning,and FO mode<PRO mode,low salinity<high salinity.The above results indicated that FO technology could be used to treat high-salinity wastewater and high-salinity wastewater containing Zn²⁺ and Ni²⁺,and the study results of different factors on water flux and the retention rate of heavy metals are helpful to the large-scale treatment of high-salinity wastewater in industrial production,thereby realizes the reduction and reutilization of high-salinity wastewater.