Smith, Jacob
(2022)
A multi-faceted approach to dye remediation with layered double hydroxides, synthesised by continuous hydrothermal method.
PhD thesis, University of Nottingham.
Abstract
Pollution of fresh water by synthetic dyes from anthropogenic sources is of increasing global concern, due to the threat they pose to both aquatic and terrestrial ecosystems. Adsorption based treatment technologies offer simple and cost-effective means to remediate industrial dye wastewater effluent. Within this work layered double hydroxides (LDHs), synthesised by continuous flow hydrothermal synthesis (CFHS), were researched to evaluate their capability as adsorbents for synthetic dye remediation.
Within the first part of this research the effect of various reaction parameters (temperature, pressure, and NaOH concentration) on Co2Al-CO3 LDH synthesis and application was investigated. This was performed, not only to discern the influence these parameters have on structural, surface, and chemical properties of Co2Al-CO3, but also to compare to previous research and determine if a generalised method for all LDHs could be developed. Modifying reaction temperature and initial NaOH concentration resulted in changes to platelet size, surface area and particle size distributions, with higher temperatures resulting in larger platelets and lower surface areas. While pressure in of itself did not influence any crystal or surface properties, lower pressures exacerbated the temperature dependence. An inverse correlation between specific surface area and CDL measurements was attributed to a reduction in exposed surface sites as platelets stack on top of each other. Comparisons with previous LDHs synthesised by CFHS method indicated that optimal reaction parameters were highly dependent on the individual LDH, making a generalised method unapplicable. Application of calcined Co2Al-CO3 for adsorption of acid orange 7 (AO7) dye revealed synthesis temperature to influence adsorption capacity, with a maximum uptake of 599 mg g-1 reported at a temperature of 50 °C.
Six LDHs, commonly utilised in literature for synthetic dye adsorption, were synthesised to determine the role of M2+ and M3+ ions on adsorption of AO7 dye. The synthesis of Ni2Al-NO3, Mg2Fe-CO3 and Ni2Fe-CO3 by CFHS was reported for the first time. AO7 adsorption was best modelled by the Langmuir isotherm for all LDHs. Kinetics analysis revealed a greater fit for all LDHs to the pseudo 2nd order model. Intra-particle diffusion modelling indicated dye adsorption to occur by both intercalation into the interlayer and sorption onto the surface. Ca2Al-NO3 and Mg2Fe-CO3 exhibited the largest maximum uptake capacities of 1586 and 1603 mg g-1 respectively. A buffering phenomenon was observed for all LDHs whereby upon suspension in water, the pH was buffered near to the LDHs point of zero charge (pHPZC), regardless of initial pH. Comparison of adsorption characteristics to physical properties of the six LDHs did not indicate correlation to any one specific physical attribute.
Due to their higher adsorption capacities, further analysis was conducted on Ca2Al-NO3 and Mg2Fe-CO3 to other dyes, to determine the effect dye properties (ionic nature, molecular size and chemical structure) have on adsorption. Maximum adsorption capacities of SY and EB onto Ca2Al-NO3 and Mg2Fe-CO3 were greatly reduced compared to AO7, attributed to their higher ionic valency and effects related to dye speciation and LDH pH buffering. Molecular size did not influence reaction kinetics, with reaction rates of AO7, SY and EB being of similar magnitude onto both Ca2Al-NO3 and Mg2Fe-CO3. Dye polarity influenced the adsorption mechanisms, with cationic MB adsorption being best modelled by the Freundlich isotherm, indicating surface adsorption to be the primary mechanism. Adsorption of cationic dyes appeared to be entirely dependent on pH effects, with Saf-O exhibiting negligible uptake onto both LDHs due to its neutral speciation at the pHPZC of both Ca2Al-NO3 and Mg2Fe-CO3.
Based on experimental conditions from previous research, Mg2Al-AO7 was successfully produced by CFHS, negating the need for traditional ion-exchange methods for AO7 intercalation post-synthesis. Intercalation of AO7 increased the interplanar spacing from 0.78 nm to 2.22 nm due to its larger molecular size compared to CO32- or NO32-ions. FTIR analysis revealed shifts in wavenumber of bonds associated with AO7 due to intercalation.
Incorporation of AO7 into LDH structure enhanced its thermal stability. Similar dye intercalated syntheses were attempted with other dyes, with successful formation of Mg2Al-SY and Mg2Al-EB. While the crystal structure was hard to discern from XRD, ordered structures and fringes of LDH platelets were observed in TEM micrographs. Synthesis of Mg2Al-AO7 attempted at lower concentrations, analogous to dye concentrations found in environmental dye effluent, were unsuccessful. Doping traditional Mg2Al-NO3 synthesis with 100 mg L-1 of dye resulted in near complete removal of AO7, SY and EB from solution. Over 90 % of the cationic dye MB was also removed by dye-doped synthesis, indicating the short residence times (~ 4s) within the CFHS reactor were sufficient for removal by surface adsorption. Dye doped Mg2Al-NO3 synthesis was just as effective at higher dye concentrations, with over 99 % AO7 removal from solution between 100 – 750 mg L-1. XRD analysis confirmed removal of AO7 occurred by intercalation mechanism at high concentrations (> 500 mg L-1), evidenced by a biphasic diffractogram of Mg2Al-NO3 and Mg2AL-AO7
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