Developing alginate-based hydrogel adsorbents for PFAS removal from water systems

The presence of per- and polyfluoroalkyl substances (PFAS) in the aquatic environment has gained notoriety over recent years as a result of their recalcitrant behavior and negative effects on human health. Granular activated carbon (GAC), which is a widely applied technology for PFAS removal from water, is limited by slow adsorption, poor uptake of short-chain PFAS compounds and competitive adsorption of other aquatic contaminants such as natural organic matter. Therefore, more efficient and greener treatment technologies that can overcome these limitations are required. While various new adsorbents have been specially designed for PFAS removal from aquatic environments in recent years, some knowledge gaps remain, including their performance in removing PFAS at environmentally relevant concentrations, desorption/adsorbent regeneration strategies and efficiency of short-chain PFAS adsorption. To overcome the remaining gaps, two materials were explored in this study as promising solutions for PFAS removal from water: rice straw-derived biochar and Moringa oleifera seed powder. PFAS have been reported to bioaccumulate in animals and plants, and their ability to bind to proteins is likely to be the main mechanism behind this. Based on that, Moringa seed powder was chosen to be tested for PFAS adsorption due to its high protein, especially albumin, content. Moringa seed albumin, which has a net positive charge, has already been successfully applied for the removal of other anionic contaminants in water and is expected to form electrostatic interactions with the anionic head-group of PFAS and undergo hydrophobic interactions with the PFAS hydrophobic chain, resulting in its removal. The other material selected to be tested in this thesis, biochar, represents a promising alternative solution for PFAS remediation given its porous structure and high specific surface area that can be easily modified to meet the required application. Rice straw was selected for producing the biochar due to its abundance and low cost. Furthermore, the conversion of rice straw into biochar has the benefit of offering a solution for waste biomass disposal. However, Moringa seed powder and biochar face disadvantages from a practical standpoint as their small particle size can cause high-pressure drop when applied in fixed bed column systems and low settling rate of particles when used in fluidized reactors. Furthermore, powder adsorbents are also difficult to separate from the treated water and are more susceptible to mass loss during regeneration. Therefore, alginate hydrogel beads were used as an immobilization matrix for Moringa seed powder and biochar to facilitate their application, offering ease of separation from treated water which can enable adsorbent regeneration and reuse. In this study, alginate hydrogel was chosen as an encapsulation matrix due to its numerous advantages, such as its simple fabrication as beads through the extrusion dripping method, biocompatibility and low cost. Therefore, the main objective of this study was to test different alginate-encapsulated adsorbents for removing two representative PFAS compounds including a short-chain (PFBS) and a long-chain PFAS (PFOS) from aqueous solutions at their relevant environmental concentrations. As PFAS analysis can be time-consuming and expensive, to assess if the Moringa-alginate beads are suitable for PFAS adsorption, preliminary experiments were conducted with humic acid as a substitute, since it shares similar characteristics with PFAS molecules of hydrophobic and anionic character. UV absorbance results, and brown coloration of the surface of the beads after the experiments confirmed the adsorption of humic acid by the Moringa-alginate beads. FTIR spectra of the beads before and after adsorption indicated that hydrophobic interactions and hydrogen bonding were the major mechanisms behind humic acid uptake and thus it was hypothesized that the Moringa-alginate beads may also have the potential to interact with PFAS through the same mechanisms. After the proof-of-concept experiments with humic acid confirmed the potential of the Moringaalginate beads for PFAS removal from water, the material was tested for PFAS adsorption through batch experiments. This work examined the influence of fabrication conditions (Moringa and alginate concentrations, needle size, etc.) on the adsorbent performance and stability. By including the Moringa powder in the alginate beads, PFAS adsorption was significantly improved. The results showed an outstanding maximum adsorption capacity (up to 942 μg g−1) for PFOS that was achieved within just 30 minutes. PFOS adsorption was unaffected by the presence of humic acid as a surrogate for natural organic matter, indicating the potential of applying the adsorbent in environmental water matrices. The outcomes of the adsorption experiments and adsorbent characterization suggested that PFOS removal was governed by hydrophobic interactions and hydrogen bonding. The Moringa– alginate beads did not perform as well for PFBS (<10% removal compared with 58% for PFOS for 100 μg L−1 solutions), which was attributed to electrostatic repulsion between the anionic functional group of PFBS and the negatively charged Moringa-alginate beads. The higher removal of PFOS was explained by its higher potential to overcome the existing electrostatic repulsion and undergo hydrophobic interactions given its longer hydrophobic tail. After demonstrating the ability of Moringa-alginate beads for removing PFOS from aqueous solution, proof-of-concept experiments were performed with albumin extracted from the Moringa seed powder and encapsulated in alginate beads to further understand the role the protein played in the removal of PFOS previously exhibited by the Moringa-alginate beads, and to determine the prospects of applying plant protein isolates for PFAS adsorption. The adsorption data from batch experiments showed an outstanding capacity of albumin-alginate beads for adsorbing PFOS, as within less than 3 hours it achieved a removal efficiency of up to 87%, demonstrating that the plant protein likely played a major role in PFOS removal by the Moringa-alginate beads. Like Moringa-alginate beads, alginate beads with encapsulated albumin also showed poor PFBS removal (<10%). The second part of this thesis evaluated the performance of rice straw-derived biochar 600 °Calginate beads for adsorbing PFAS from aqueous solutions. Three different biochar composites were produced with non-treated biochar (NT), or biochar pretreated with either diammonium phosphate (DAP) or ammonium sulfate (AS). The pretreatment with DAP and AS was applied as a way of improving the cost effectiveness of biochar production as it affects the degradation of biomass during pyrolysis increasing the char yield. The biochar-alginate beads proved their potential for PFAS uptake from water as within less than 16 hours maximum removals of up to 99% of PFOS for NT biochar 600 °C-alginate beads and nearly 40% of PFBS for the DAP biochar 600 °C-alginate beads were obtained, which were higher than some natural material-based adsorbents reported in some previous studies. Adsorption with the three different biochar composites did not depend on the pH and was practically unaffected by the presence of humic acid as surrogate for natural organic matter. Adsorption was mostly attributed to hydrophobic interactions. The pyrolytic temperature can change the characteristics of biochar such as surface charge, pore size distribution, total surface area and elemental composition. Based on that, this thesis further explored the application of a higher pyrolytic temperature (900 °C cf. 600 °C) on biochar production aiming to improve the performance of biochar-alginate beads for removing PFAS from aqueous solutions. The influence of the higher pyrolysis temperature on the biocharcomposites for PFAS adsorption was then evaluated with NT biochar 900 °C-alginate beads for PFOS removal and DAP biochar 900 °C-alginate beads for PFBS removal. The maximum adsorption capacity of the NT biochar-alginate beads for PFOS was superior to the biochar prepared at 600 °C (2947 μg g−1 vs 1572 μg g−1), which was attributed to its higher hydrophobicity and surface area. In contrast, the removal efficiency for PFBS remained low regardless of the pyrolysis temperature, as the adsorbent surface remained negatively charged which likely suppressed PFBS adsorption due to electrostatic repulsion. Given the satisfactory performance of NT biochar 900 °C-alginate beads for PFOS adsorption, evaluation of its dynamic adsorption properties was carried out with PFOS using a small-scale column system. The column experiments showed the dependency of the breakthrough curve on the flow rate and initial concentration of PFOS, with breakthrough time declining with increasing the initial PFOS concentration and flow rate. PFOS was desorbed from the adsorbent within only 30 minutes after chemical regeneration by applying either ethanol (50% and 100%, v/v) or methanol (50% and 100%, v/v). A desorption efficiency of around 90% was achieved when 100% methanol was tested and 80% when 100% ethanol was tested. Despite the initial drop in the removal efficiency of the regenerated adsorbent after the first cycle, its adsorption performance over the next two cycles remained relatively stable. Among the tested adsorbents, biochar-alginate beads showed the highest potential for PFAS removal from water. Further optimization of the fabrication of the adsorbents for improved removal of short chain PFAS is still necessary. Chemical modification is a possible path for increasing the surface area and positive charge density of the biochar as well as creating new functional groups, which all can be beneficial for PFBS uptake. Future work focused on further optimization of biochar production by testing different temperatures and residence times is also necessary. The effect of water matrix constituents on the breakthrough curves also needs to be assessed. Overall, this study contributed to the understanding of the mechanisms behind PFAS adsorption and gave insights into the prospects of applying novel and more environmentally friendly adsorbents for the remediation of PFAS-contaminated water which would support the design of more effective adsorbents for PFAS remediation in the future.