Johnston, Amy-Louise
(2024)
Exploring the use of continuously synthesised layered double hydroxides for the sorption of pharmaceuticals from wastewater.
PhD thesis, University of Nottingham.
Abstract
Anthropogenic activities are resulting in significant pollution of global environmental waters, causing detrimental impacts to human and environmental health. Emerging contaminants (ECs) are one class of pollutants of which detection in the environment is rapidly growing, with adverse and often unknown effects. Pharmaceuticals, including antibiotics, fall within this class of pollutants, and their presence in the environment is additionally concerning due to their contribution to the spread of antimicrobial resistance. Current wastewater treatment technologies were never designed for the removal of ECs, therefore advanced wastewater treatment technologies need to be implemented. Sorption is a promising technology.
Layered double hydroxides (LDHs) are emerging as attractive sorbent materials, due to their low-cost and simple synthesis. LDHs have tuneable physiochemical properties which can be harnessed to design bespoke high-performing sorbent materials. Whilst a large body of literature is available on LDH performance for the removal of conventional pollutants (dyes, metals) from water, the removal of pharmaceuticals is less commonly explored. A case study, evaluating literature regarding the sorption of a common dye (methyl orange) by LDH sorbent materials determined that the complex combination of physiochemical properties of LDHs influences sorption performance of the pollutant, methyl orange from water matrices. This is likely due to the synergistic sorption mechanisms occurring (i.e. surface sorption and anion exchange) for the removal of organic pollutants. Environmental factors, plus the physiochemical properties of the pollutant, also influence LDH sorption performance. Further, studies considering the removal of organic pollutants at realistic environmental concentrations and from complex water matrices are limited.
This research explores the performance of continuously synthesised LDHs for the removal of various antibiotics from water matrices. For the first time MgxFe-NO3-LDH and MgAlFe-NO3-LDH are synthesised using a continuous hydrothermal flow reactor. The stability of four different LDHs (Mg2Al-NO3; Mg4Al-NO3; Mg2Al-CO3; Mg4Fe-NO3) in laboratory-grade water are investigated, including release of metals, nitrate anions, and change in solution pH. Changes in water composition were monitored as they influence LDH sorption performance. Characterisation by pXRD and FT-IR show LDHs to be suitably stable in water for use as sorbent materials, aligning with literature reports.
The sorption of two antibiotics, sulfamethoxazole (SMX) and amoxicillin (AMX), by a Mg2Al-NO¬3-LDH are reported in detail. For the first time, experimental conditions including starting concentrations of antibiotics at environmentally relevant concentrations (<mg/L) and removal from municipal wastewater effluent (WWE) are explored. Experimental parameters (i.e. temperature, pollutant concentration) were selected to ensure the research supports moving up Technology Readiness Levels, and hence future application. The importance of robust pollutant quantification, especially when using sensitive techniques such as liquid chromatography tandem mass spectrometry (LCMS/MS) is discussed. Aspects to conducting LCMS/MS analysis, such as using matrix matched calibrations curves and evaluating analyte carryover, are shown to impact the calculation of sorption capacity of sorbent materials. In addition, experimental design is discussed including monitoring degradation of pollutants in control experiments, agitation methods and experimental reproducibility. Removal percentage (%) of AMX from laboratory-grade water is not found to be significantly different between LDH synthesised in different batches (N=2), across different experiments (N=3) (p>0.05).
The sorption of SMX by Mg2Al-NO3-LDH from laboratory-grade water was shown for the first time to exhibit an unusual sorption/desorption behaviour. Whilst a maximum SMX removal percentage is found to be 35.6 %, this was followed by gradual SMX desorption. Therefore, no equilibrium removal of SMX is observed, with a removal percentage of 0.4 % at 48 h. Desorption is demonstrated not to be due to changes in solution pH, but rather proposed to result from the competition with nitrate ions released from the LDH material itself. Further, no sorption of SMX was observed when using WWE as the water matrix over 24 h. This was due to competition for sorption sites from naturally occurring components present in the WWE. In contrast, AMX removal (>95 %) is observed from laboratory-grade water within 24 h under a variety of experimental conditions. However, as found for SMX, no sorption of AMX is observed from WWE. Instead, degradation of AMX is observed in WWE with and without the presence of LDH, with the AMX concentration at 24 h significantly lower in the control experiment compared to when LDH was present (p<0.05). It is shown that Mg2Al-NO¬3-LDH preferentially sorbs naturally present metals in the WWE, with the concentration of Zn and Mn decreasing by 85 % and 79 % respectively, which in turn impacts the stability of AMX in WWE.
The research is expanded to evaluate the sorption performance of LDH with different compositions (Mg4Al-NO3; Mg2Al-CO3; Mg4Fe-NO3). No improvement was found for the removal of SMX compared to Mg2Al-NO¬3-LDH, as no equilibrium removal (removal percentage <2 % at 24 h) was observed for any of the LDH. For AMX different removal percentages, ranging from 10-39 %, were found for the various LDHs, hence none show an improved performance compared to Mg2Al-NO¬3-LDH. The difference in sorption performance between different LDH materials aligns with literature reports of variable LDH sorption performance not clearly correlated to LDH properties such as specific surface area. Finally, a degree of prediction capability for sorption and different pharmaceutical pollutants can be observed, appearing to be related to the speciation of the pollutant. Oxytetracycline, anionic in solution, was found to be removed by Mg2Al-NO3-LDH, reaching removal percentage of 92%. Whereas propranolol, a cation in solution, was not removed by Mg2Al-NO3-LDH, due to the inability to form favourable electrostatic interactions or undergo anion exchange mechanisms.
Overall, this work contributes to current state-of-the-art knowledge on the use of LDHs as sorbent materials as an advanced wastewater treatment technology. Sorption is greatly influenced by water matrix composition, which emphasises the importance of conducting research into sorbent material performance in environmental waters early in development. Further, this work reinforces the understanding that the relationship between the sorption performance and physiochemical properties of LDH is multifaceted, resulting in complex ideal LDH properties. Future work should focus on routes to predict LDH sorption performance, including understanding the preferential sorption of different pollutants by LDH from complex water matrices. Developing a deeper understanding of the relationship between sorption performance and physiochemical properties for LDH materials would allow for the design of bespoke sorbent materials. Ultimately this would contribute to LDH sorbent materials finding application as an advanced wastewater treatment technology.
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