| Membrane purification technology had been used in the practical water treatment processes due to its high separation performance,stable water product quality,low operation cost and small carbon footprint.However,the ultrafiltration membrane fouling in ultrafiltration technology and the trade-off between“selectivity and permeability”of nanofiltration membrane in nanofiltration technology are two important problems that limit the wide popularization of membrane purification technology.In practical application,carbon materials such as powder activated carbon（PAC）,graphene oxide（GO）,carbon nanotube（CNT）and so on,are usually used to solve the above problems and achieve certain results,but there are still many deficiencies such as low efficiency or high material cost that need to be improved.Therefore,in this study,crayfish shell biochar（CFS）,a carbon material with simple production,low cost and unique properties,was used to deal with the problem of the ultrafiltration membrane fouling and the trade-off between“selectivity and permeability”of nanofiltration membrane in membrane purification technology,and its enhancement efficiency and mechanism was studied.The mitigation performance and mechanism of CFS biochar under different pyrolysis temperatures on typical dissolved organic matter(Humic acid（HA）and polysaccharide（SA）ultrafiltration membrane fouling were firstly studied.Results indicated that high-temperature biochars（CFS800-CFS1000）have more advantages in alleviating membrane fouling than that low-and medium-temperature biochars（CFS300-CFS700）,especially for the irreversible membrane fouling induced by HA and reversible membrane fouling induced by SA.After pretreatment with 1 g/L of CFS800,the final normalized membrane flux（J/J0）of HA and SA increased from 0.315and 0.0875 to 0.819 and 0.620,respectively.The membrane fouling mitigation performance was twice more than that of PAC under the same conditions（0.491 for HA and 0264 for SA）.Further analysis showed that the high adsorption capacity of high-temperature biochar for HA（>80%）was the main reason for the mitigation of irreversible membrane fouling of HA（57.3%to 6.75%）.While the reason for mitigation of reversible membrane fouling of SA were,on the one hand,the cross-linking reaction between the hydrogel calcium component of high-temperature biochar and the SA molecule,which could quickly remove part of SA from the raw water.On the other hand,the hydrogel flocs produced by cross-linking reaction would eventually accumulate on the surface of the PVDF membrane,and form an amorphous hydrogel layer with the ability to intercept subsequent pollutants.Based on the above characteristics of cross-linking reaction between high-temperature biochar and SA,the ball-milled CFS800（BCFS800）was subsequently doped into the separation layer of conventional hydrogel nanofiltration membrane（Ca-SA membrane,the comparison membrane）,and the CFS composite nanofiltration membrane（the composite membrane）with high selectivity for dye molecules/inorganic salt ions was obtained.Taking the congo red molecule/sodium chloride ion mixture as an example,results indicated that the selectivity（149.2）,permeation flux（6.5 LMH/bar）and compressive ability of the composite membrane were higher than those of the comparison membrane.Further analysis showed that the doping of BCFS800 improves the adsorption capacity of the composite membrane for congo red,resulting in the higher interception ability of the composite membrane than that of the comparison membrane.Meanwhile,the porous structure of BCFS800 gave more water molecules and inorganic salt ion channels in the separation layer of the composite membrane,which improved the permeation flux and decresead the sodium chloride ion interception rate（4.54%vs 7.56%）.Moreover,during 48 h operation,the composite membrane always maintained a high Congo red molecular rejection rate（>97.5%）and a low sodium chloride ion rejection rate（<5%）,and a stable separation performance.In addition,the final flux of the fouled composite membrane was still higher than that of the fouled comparison membrane.In summary,compared to the comparison membrane,BCFS800 could enhance the selectivity of the composite hydrogel nanofiltration membrane,and was more suitable for the separation of dye molecules/inorganic salt ions.To further improve the flux of biochar doped nanofiltration membrane,BCFS800combined with polydopamine（PDA）was finally applied to thin-film composite（TFC）nanofiltration membrane,and the PDA@TFC membrane with high-flux separation ability for tetracyclic antibiotic wastewater was obtained by the bar coating method.Results indicated that the appropriate concentration of BCFS800 suspension（1～4mg/m L）could ensure the integrity of the modified membrane structure layer and improved the permeation flux and interception ability.Excessive（6～8 mg/m L）concentration of BCFS800 suspension would break the membrane separation layer and affected the interception performance of the PDA@TFC-6 and PDA@TFC-8.In addition,compared to the pristine PDA@TFC-0,the optimized PDA@TFC-2membrane had the highest interception efficiency for antibiotics（91.9 for chlortetracycline,90.3%for tetracycline,91.3%for oxytetracycline and 81.8%for ciprofloxacin）,and also had the highest permeation flux（the pure water flux was increased from 18.6 LMH/bar to 32.0 LMH/bar）.Meanwhile,PDA@TFC-2 also had a good cycle performance,after five times of“oxytetracycline filtration-sodium hydroxide cleaning”,the pure water and feed water flux of the regenerated PDA@TFC-2 membrane could still maintain 70.6%and 34.1%of that of the fresh membrane,and the interception rate of oxytetracycline was more than 60%.In summary,the findings of this study not only alleviated the ultrafiltration membrane fouling and the trade-off between"selectivity and permeability"of nanofiltration membrane,but also broadened the application scope and expanded the use-value of biochar to a certain extent,thus had strong engineering practical application value. |
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