Seawater Distillation And Condensed Water Quality Control Based On Air-water Interfacial Solar Heating

Recently,a kind of new method for seawater desalination based on 'interfacial solar heating' has triggered significant research interests and been regarded as one of the effective approaches to solve the global water crisis.However,the volatile organic compounds in the source water will unavoidably evaporate along with the water steam because of the high temperature during the solar distillation process and then pollute the condensed water,which may pose a great risk to the safety of water quality.In this work,a carbon nanotube based floating solar still(CNTs-FSS)and a g-C3N4/MoS2 based floating solar still(CM-FSS)were designed and fabricated,with the carbon nanotubes(CNTs)and g-C3N4/MoS2 hybrids as the photothermal materials,air-laid paper(ALP)as the water transport channel,polyurethane sponge(PUS)and expandable polyethylene(EPE)foam as the floatable insulation materials and sodium persulfate(PS)as auxiliary oxidant.Also,their water evaporation rate and removal capacity of typical organic pollutants were investigated.In these floating solar stills:the ALP will work as the water transport channel to deliver seawater to the photothermal materials;the photothermal materials will absorb the solar energy to heat the seawater and remove the organic pollutants by their adsorption properties,visible-light photocatalytic activity or catalytic oxidation of sodium persulfate activated by MoS2 and high temperature of the air-water interface during the solar distillation process;the floatable insulation materials will prevent the heat from being transferred from the air-water interface to the underlying water.CNTs-FSS showed high water evaporation rate and excellent removal capacity of organic pollutants by its adsorption properties.The water evaporation rate of the CNTs-FSS,having a 97.33%of optical absorption(wavelength from 300 to 2000 nm),could be calculated to 0.88 kg/m2·h under solar irradiance with intensity of 1 sun for 1 h,which is 2.80 times higher than that of pure water.Besides,when carbamazepine,nitrobenzene,and naproxen were chosen as the organic pollutants,the concentrations of the three organic pollutants in the condensed water were 29.4%,42.4%and 4.3%of their initial concentrations respectively after solar distillation without CNTs-FSS.However,the concentrations of the three organic pollutants in the condensed water were reduced to 10.2%,17.1%and 0.5%of their initial concentrations respectively after solar distillation with CNTs-FSS.Similarly,CM-FSS also showed higher water evaporation rate and excellent removal capacity of organic pollutants by its adsorption properties and visible-light photocatalytic activity.The water evaporation rate of the CM-FSS,having a 96.19%of optical absorption(wavelength from 300 to 2000 nm),could be calculated to 1.23 kg/m2·h under solar irradiance with intensity of 1 sun for 1 h,which is 4.09 times higher than that of pure water.Besides,when carbamazepine,nitrobenzene,and rhodamine B with initial concentrations of 500.0 ?g/L,500.0 ?g/L,and 50 mg/L respectively were chosen as the typical organic pollutants,their concentrations in the condensed water decreased significantly to 235.3?g/L,236.2 ?g/L and 0.9 mg/L respectively after solar distillation with CM-FSS.After adding sodium persulfate(PS)to the distillation system,the concentrations of the three organic pollutants decreased more obviously,even completely removed.Besides,the water evaporation rate of the CNTs-FSS floating on pure water and seawater for 30 days could be 1.54-1.58 kg/m2·h and 1.50-1.57 kg/m2·h respectively under solar irradiance with intensity of 2 sun and the water evaporation rate of the CM-FSS floating on pure water for 30 days could be 1.15-1.23 kg/m2·h under solar irradiance with intensity of 1 sun.Therefore,both the CNTs-FSS and CM-FSS showed outstanding stability and durability for seawater desalination.Finally,when real seawater samples obtained from the East China Sea were used as source water for solar distillation with CNTs-FSS and CM-FSS,the drinking water quality indexes such as conductivity,cations(Na+,K+,Ca2+,Mg2+)and anions(F-,Cl-,NO3-,SO42-)in condensed water were all below the values defined by the World Health Organization(WHO),the US Environmental Protection Agency(EPA)and the standards for drinking water quality in China(GB 5749-2006).