Adsorbents for the removal of organic and inorganic contaminants from water

Abstract

The molluscicide metaldehyde has been frequently detected in ground and surface waters in concentrations exceeding the 0.1 μg L-1 EU regulatory limit, as industrial Granular Activated Carbon (GAC) has been proven inadequate in achieving efficient removal by adsorption. In the present work, Chapter 3.1 describes our research efforts to identify better adsorbents for metaldehyde. Several HCPs and Polymers of Intrinsic Microporosity (PIMs) were synthesised and tested for their metaldehyde adsorptive properties through batch equilibrium experiments. Upon comparison to benchmark adsorbents, a cost-effective HCP based on the TPB aromatic monomer, polymerised in the presence of AlCl3 and DCM, was identified as an excellent candidate for aqueous metaldehyde removal. Novel externally crosslinked analogues of the material were found to display enhanced adsorptive properties, and this was shown by gas adsorption data to be due a result of higher meso and macroporosities, which result in efficient intraparticle mass transport of metaldehyde to the micropores. Solid-state 13C NMR spectroscopy confirmed that the externally crosslinked networks contained a higher degree of alkyl-based linking units, in comparison to TPB-DCM for which Scholl coupling was prevalent. Further aqueous experiments including isotherm studies, adsorption kinetics and Rapid Small Scale Column Tests (RSSCTs) for the novel network derived from TPB & trimethylorthoformate (TMOF) crosslinker are discussed in Chapter 3.2. The overall findings suggest an exceptional maximum adsorption capacity for metaldehyde qmax = 125.00 ± 6.03 mg L-1, as determined from the Langmuir model, very fast kinetics following pseudosecond order kinetics with K2 = 0.15 ± 0.02 mg-1 g min-1, and excellent efficiency in RSSCTs, with 2500 column volumes of 0.5 mg L-1 metaldehyde-spiked water purified before breakthrough. Regeneration of the adsorbent thermally, and via solvent wash were shown to be possible but were not fully investigated. Pre-polymerisation modification of the TPB unit by the introduction of methoxy and hydroxy substituents was found to negatively affect the porosity and metaldehyde uptake of the materials produced, whereas no significant differences were observed for fluorine substituents. A magnetically responsive nanocomposite of TPB-HCP and functionalised magnetite particles was synthesised successfully and retained high metaldehyde affinity.

Chapter 3.3 describes that synthesis of sulfonated HCPs and PIMs, through post-polymerisation modification routes, based on acetylsulfuric acid (ASA) and sulfuric acid/triflic anhydride (SA-TfA). The materials were characterised by SS ¹³C NMR, TGA and IR, which confirmed successful sulfonation, and the ion-exchange capacity (IEC) was determined by Boehm titration methods. The degree of sulfonation was found to greatly enhance Pb²⁺ uptake properties, despite the significant decreases in pore volumes and BET surface areas. Highly sulfonated adsorbents based on TPB, triptycene (tryp) and dibenzomethanopentace (DBMP) units, with IECs ranging between 4.2-6.5 mmol g⁻¹, were found to achieve similar removal efficiencies approaching 99% at initial Pb²⁺ concentration of 160 mg L⁻¹ and adsorbent dose of 1 mg L⁻¹. Isotherm studies on the DBMP-DCM/SA-TfA modified adsorbent further demonstrated the high affinity of the material for Pb2+, with the dataset showing a satisfactory fit for both Freundlich and Langmuir models, and excellent fit to the Dubinin-Astakov model, suggesting that a chemisorption process is prevalent. This was further supported by the Langmuir separation factor, as well as the observed shifts in Pb²⁺-induced sulfonic acid vibrational wavenumbers.

Chapter 3.4 investigates the degradation of metaldehyde using the derived sulfonated adsorbents, through batch-type GC-MS and NMR experiments. A fast decline in the kinetics of the process was attributed to the narrow pore size distribution of TPB-DCM/ASA, in addition to the strong binding of metaldehyde degradation products to the material, which compete with metaldehyde for the catalytic sites. The breakdown of metaldehyde to acetaldehyde was observed by NMR, which showed additional conversion of acetaldehyde to a variety of products. The presence of paraldehyde, acetic acid, formic acid, acetone, CO₂ was observed in addition to poly(oxymethylene) glycol chains of various lengths. At high contact times, the presence of alkanes, olefins and various other unidentified compounds was confirmed by the ¹H-¹³C HSQC spectrum. It is speculated that the conversion is likely proceeding through aldol condensation to form acetaldol which forms crotonaldehyde upon dehydration, however, the high complexity of the redox conversion cycle did not allow for full mechanistic determination. The versatile catalytic activity of the adsorbent TPB-DCM/ASA is recognised and should be further investigated.