Polyvinylidene Fluoride Membrane May Alleviate Freshwater Crisis

Polyvinylidene fluoride is a highly non-reactive thermoplastic fluoropolymer polymerized from vinylidene fluoride. Its chemical formula is (C₂H₂F₂)_n.

The shortage of freshwater resources has increasingly become a serious challenge facing the world. Using clean solar energy to heat, evaporate, and condense seawater is a new way to obtain clean freshwater resources. The solar interface heating technology developed in recent years uses photothermal conversion materials to directly and efficiently convert absorbed solar energy into thermal energy, and collects it at the air/water interface, which can effectively prevent heat from diffusing into the water body. Natural wood and anodized aluminum membranes have been tried for seawater desalination. However, these materials are difficult to process, difficult to prepare on a large scale, and difficult to control the pore structure and performance, and still have limitations in practical applications. Directly using commercial porous polymer films as substrates and subjecting them to photothermal functional modification to manufacture solar evaporators is considered a solution with large-scale application prospects. Taking the polyvinylidene fluoride (PVDF) membrane with excellent stability and high degree of commercialization as an example, the dual continuous pores obtained by the phase separation method are not conducive to moisture transfer. Therefore, regulating the pore structure of polymer membranes and accelerating water transfer and evaporation efficiency are the keys to developing high-performance solar evaporation membrane materials.

The PVDF membrane prepared by some researchers has a through-pore structure and asymmetric tapered pore characteristics. Under the control of CuSO₄/H₂O₂, hydrophilic polydopamine nanocoating is gradually formed on the membrane pore wall, and finally a membrane with asymmetric hydrophilic/hydrophobic properties is obtained.

The researchers placed the hydrophilic side of the membrane in contact with water, while the other side was hydrophobic, allowing it to float on the water. The hydrophobic layer can convert solar energy into heat energy in situ, while the hydrophilic channels can continuously pump water to the interface for evaporation. Utilizing the multi-level scattering effect of the tapered holes, the film can increase the solar energy utilization efficiency to 97%. Under one solar irradiation (1 kW·m^-2), the surface temperature of the film quickly rises to 41°C. At the same time, the unique straight-through pore structure of the membrane provides the most direct channel for water transfer and steam escape, enabling a water evaporation rate of up to 1.08 kg·m^-2·h^-1. The research team further compounded the film with insulating polyurethane foam to prepare a solar seawater desalination device. The low thermal conductivity of the foam greatly inhibits the diffusion of heat into the water body, reduces heat loss, and increases the photothermal conversion efficiency to 90.2%. At the same time, the absorbent paper sandwiched between the film and the foam can transport water to the surface of the film for evaporation, with a rate of 1.58 kg·m^-2·h^-1. After the device desalinizes seawater, the concentrations of Na⁺, K⁺, Mg²⁺, Ca²⁺ and B³⁺ in the water are reduced to less than 1 ppm, which is in line with the drinking water standards promulgated by the World Health Organization.

The experimental results strongly prove the excellent performance of through-hole membrane photothermal materials in seawater desalination, providing a green and efficient solution to alleviate the freshwater crisis. The membrane preparation process is simple and controllable, and has broad prospects for large-scale application.

Reference

1. Janus poly (vinylidene fluoride) membranes with penetrative pores for photothermal desalination.
   Research (2020).