The Efficiency And Mechanism Of Gravity-driven Filtration System Using Woody Membrane To Remove Heavy Metals In Surface Water

Heavy metal ions in surface water pose a threat to drinking water safety.And traditional methods used for the removal of heavy metals are also found highly inefficient owing to their several limitations.Gravity-driven membrane（GDM）filtration process with the advantages of low energy consumption and simple operation at ultra-low-pressure,has been used in drinking water treatment,but its ability to remove heavy metal ions needs to be improved.As a green,easily available,and inexpensive material,wood has a high affinity for heavy metals,however,little research has been reported on the application of wood membranes in GDM systems.In this study,the efficiency of wood membranes in removing Fe,Mn and Cu from surface water was investigated by combining them with a GDM system.Meanwhile,the mechanism of the system was elucidated by analyzing the characteristics of the membrane and the filter cake layer on the membrane after operation,with the aim of improving the removal rate of heavy metals in surface water and improving drinking water quality in an environmentally friendly and inexpensive way.The surface water was from Jingmi River（Beijing,China）originating from the Miyun Reservoir and supplying drinking water to Beijing city.The raw water with Fe,Mn,and Cu concentrations of～200μg·L^-1 was prepared by adding Fe Cl₃、Mn Cl₂ and Cu Cl₂.During the long-term operation of two GDM systems,the wood membranes displayed average removal rates of 52%,27%,and 52%for Fe,Mn,and Cu,respectively,while the polymer membranes showed average removal rates of 30%,16%,and 40%,respectively.The X-ray photoelectron spectra（XPS）indicated that the percentages of Fe,Mn and Cu in the cake layer of wood membrane were 0.61%、0.56%、0.57%,respectively,which were higher than those in the polymer membrane cake layer（0.18%、0.16%、0.16%）.The Fourier transform infrared spectrometer（FTIR）showed that a higher content of carboxyl（-COOH）groups were observed in the cake layer of the wood membrane GDM system as compared to the polymer membrane GDM system.The-COOH groups can exchange ions with metal ions,resulting into the improved removal of heavy metals.Meanwhile,the high-throughput sequencing results also showed the higher presence of the relative abundance of microbial communities capable of removing heavy metals in the biofilm of wood membranes,including Proteobacteria,Acidobacteriota and manganese oxidizing bacteria.Wood membrane GDM system has higher heavy metal removal rates.In order to improve the flux of the wood membrane GDM system and take advantage of its high removal ability,the wood membrane was coupled with a polymer membrane to remove metal ions in surface water.The quality of raw water was the same as described above.The results showed that the stable flux of GDM system having wood membrane coupled with polymer membrane（GDM1）was 22%higher as compared to the polymer membrane GDM system alone（GDM2）.The average removal rate of Fe,Mn,and Cu was 67%,43%,and 59%for GDM1,compared to 64%,15%,and 36%for the polymer membrane alone（GDM2）,respectively.The FTIR results showed the highest intensity of the-OH peak in the cake layer of the wood membrane（GDM1-WM）and the C=O peak in the cake layer of its polymer membrane（GDM1-MF）for GDM1.The-OH and-COOH groups can serve as binding sites for metal ions,thus increasing the removal of Fe,Mn,and Cu from the solution in GDM1.Moreover,the concentrations of Fe,Mn,and Cu in both GDM1-WM(4.0,0.93 and 3.80 mg·m^-2)and GDM1-MF(29.29,4.25 and 87.67 mg·m^-2)were found higher than the GDM2-MF(25.67,4.52 and 29.19 mg·m^-2).This indicated the positive role of wood membrane for heavy metal removal in GDM1.The Three-Dimensional Excitation Emission Matrix（EEM）of EPS in GDM1-WM showed the different peaks of humic substances from those of the GDM1-MF and GDM2-MF.Moreover,the presence of humic substances on the surface or inside the wood biofilm could also interact with metal ions and form colloids,which further facilitated the removal of heavy metals.High-throughput sequencing results showed that the relative abundance of microbial communities capable of removing heavy metals in GDM1-MF was higher,including Flavobacterium,Acidobacteriota and manganese oxidizing bacteria.This also enhanced the GDM ability for heavy metal removal.Therefore,the GDM system with wood membranes coupled to polymer membranes not only improves the stable flux of the system,but also maintains a high retention of heavy metals,due to the formation of unique cake layer.