Degree

Doctor of Philosophy (PhD)

Department

Department of Civil and Environmental Engineering

Document Type

Dissertation

Abstract

Addressing global freshwater scarcity is a pressing challenge, and desalination has emerged as a viable solution to generate high-quality water without compromising natural freshwater ecosystems. In comparison to conventional desalination methods such as thermal distillation and membrane-based processes, hydrogels offer distinct advantages including strong water absorption, water retention, well-defined pore structures, customizable functional groups, and responsive properties to external stimuli. These unique characteristics position hydrogels as promising candidates for water treatment applications. Hydrogel-based desalination involves the utilization of hydrogels to absorb water from saltwater, and then discharge low-concentration solutions to obtain reclaimed water. The fundamental principle underlying the desalination ability of charged ionic hydrogels lies in their ability to impede the entry of partial salt ions from the bulk solution into the charged hydrogel, owing to the presence of charged ionic groups within the hydrogel matrix. However, due to the excellent water retention of hydrogels, dewatering of hydrogels has become a great challenge. Currently, the most common dewatering methods include mechanical squeezing, electric field stimulation, and heating, but these methods usually have low water recovery, weak stability, or high energy consumption. Therefore, efficient and stable hydrogel dewatering methods are worthy of further research and development.

In this study, a variety of stimuli-responsive hydrogels were synthesized and applied to study the dewatering of swollen hydrogels under different stimuli. First, ammonium bicarbonate (NH4HCO3) solution was introduced for the dewatering of swollen poly (acrylic acid-co-acrylamide) (PAAM) hydrogels due to their high osmotic pressure. This method resulted in nearly 48% water recovery and 58% salt rejection in 0.85 g/L salt solutions. Additionally, using NH4HCO3 solution as a draw agent was very gentle and stable with consistent water recovery over more than 20 cycles in 0.85 g/L and 5 g/L NaCl solution. Inspired by NH4HCO3, a novel method utilizing ammonia gas (NH3) with a higher osmotic pressure was tested to dewater PAAM hydrogels for desalination, yielding remarkable results. The system achieved up to 90% water recovery for over 15 g/L NaCl solution and even real seawater. More notably, the system was extremely stable, with no apparent changes observed in water recovery over 100 cycles in 1 g/L, 15 g/L NaCl solution, and real seawater. For these two experiments, we calculated the theoretical value of the salt concentration that the hydrogel could repel when absorbing saline by Donnan's theory. The osmotic pressures of NH4HCO3, NH3, NaCl, and hydrogel were also simulated by software, which provided a theoretical basis for hydrogel dewatering and salt repellency. In addition, we also used a series of characterizations, such as SEM, SEM-EDS, ATR-FTIR, XPS, and DSC, to spy on the internal structure of the hydrogel, so as to better understood and regulate its characteristics in saltwater desalination. Considering the natural charging characteristics of hydrogel, we thought of using the electric field to stimulate hydrogel to dewater so as to achieve the purpose of saltwater desalination. In the third study, the electric field was introduced to stimulate the swollen (graphene nanoplatelets)-poly (2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamide) (GNP/PAMPSAM) hydrogels with excellent conductivity. With the optimal amount of GNP added, the electrical conductivity of the PAMPSAM hydrogel was increased by 3 times, and the fastest dewatering could be achieved at a voltage of 10 V. This method had over 80% water recovery and salt rejection. The addition of GNP not only greatly enhanced the conductivity of the hydrogel, but also enhanced the swelling ability and stability of the hydrogel to a certain extent. After 5 cycles of experiments, the hydrogel still maintained its desalination ability. The above methods all needed to consume a certain amount of energy to remove/recover NH4HCO3, NH3 or drive an electric field. We thought of using clean energy, solar energy, to desalinate saltwater from the hydrogel. In the fourth study, we also used the GNP hybrid hydrogel GNP/PAMPSAM. Under one sun-intensity irradiation, the hydrogel recovered more than 95% of the absorbed water within one hour, and had a salt rejection of more than 95%. The evaporation rate of this hydrogel reached nearly 4 kg/m2/h, and produced 30 L/m2 of product water per day. GNP could improve the capillary structure inside the hydrogel, greatly enhancing the water escape ability and photothermal conversion ability of the hydrogel. It was confirmed that the GNP/PAMPSAM hydrogel was an excellent water evaporation platform.

Date

7-11-2023

Committee Chair

Dr. Samuel Snow

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Available for download on Friday, July 10, 2026

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