Clark, Ian
(2018)
Continuous synthesis and characterisation of layered double hydroxide nanomaterials for their application for dye wastewater remediation.
PhD thesis, University of Nottingham.
Abstract
The first part of this research was aimed to improve continuous synthesis of LDHs through a matrix of experiments that investigated the influence of key factors affecting synthesis. These factors; temperature, pressure and reactant concentration were investigated for Ca2Al-NO3 LDHs. Alteration of reaction temperature and pressure caused changes to crystal domain length (CDL). As temperature was increased, tune-ability through pressure manipulation was lessened. The effect of the NaOH concentration in the precipitation reaction was found to have an impact on CDL. The highest NaOH concentration was found to produce smaller domain lengths. The initial excess of OH- causes nucleation of crystals to occur preferentially over sustained growth. High NaOH content caused Ca(OH)2 and Ca2Al-CO3 contamination, due to an excess of Ca2+ in the precursors and excess NaOH in the reaction. The presence of the impurities alter the chemical makeup of the compounds and the thermal properties. The effect of the changes to CDL was not realised in the other physical characteristics of the LDH, as platelet size remained at an average 2-5μm with no trend corresponding to CDL variations. Specific surface area (SBET) also showed no change in relation to CDL variation or the modification of reaction temperature, pressure or precursor NaOH.
Scale up synthesis of LDHs focused on Zn2Al-CO3. Reactions to produce Zn2Al-CO3 were carried out at bench, pilot and industrial scale. All variables were kept constant, bar the reactor size and the flow rate. The pilot scale reactor operated at 20-30 times greater than bench scale. The pilot scale LDH exhibited greater CDL, this was attributed primarily to a settling period post-reaction, where the suspension was allowed to settle in the reaction liquor. Platelet diameter increased from 120nm at bench to 177nm at pilot scale and 165nm at industrial scale. The changes to particle size do not correspond with changes to SBET. Surface area decreased from 58m2 g-1 to 50m2 g-1 between bench and pilot synthesis but the surface area was recorded at 64,1m2 g-1 for the industrial Zn2Al-CO3 Industrial synthesis produced a Zn2Al-CO3 LDH with CDL (19nm) comparable to the bench scale sample (18nm) but smaller than the pilot scale sample (26nm).
Zn2Al-CO3 LDHs for remediation of a synthetic dye was carried out on two types of reactive dye. The LDH was calcined and the resulting mixed metal oxide (MMO) was characterised. Calcination caused an increase in SBET from 50m2 g-1 to 57m2 g-1. Wet storage of the MMO reformed the LDH crystal phase, partly after 2 weeks and completely after 6 weeks. Adsorption of dye was best modelled by the Langmuir isotherm. The fit of the isotherm model was reduced as adsorption temperature was increased, due to inhibited uptake of dye at low initial concentration (C0). The maximum uptake capacity (qm) at 293K, was 589mg g-1 and 895mg g-1 for Reactive Black 5 and Reactive Orange 16, respectively. Adsorption was also modelled kinetically by a pseudo 2nd order model. Investigation of the impact of competing anions found that, Cl- had no effect on adsorption of either dye molecule, however addition of Na2CO3 and Na2SO4 inhibited adsorption. Thermal regeneration of the adsorbent significantly reduced the adsorption capacity in subsequent cycles. Washing with Na2CO3 prior to re-calcination mitigated the effect of regeneration. Breakthrough curves were fit with the Yoon-Nelson and Thomas models best, while the Adams-Bohart model was not appropriate. The adsorption capacity (qe) from the Thomas model was between 7.2mg g-1 and 7.6mg g-1, which is considerably lower compared with the batch scale Langmuir qm value. The reduction is attributed to a number of factors. The empty bed contact time was not enough to allow the adsorption to occur completely and breakthrough occurred after the first, timed sample. The presence of the PMMA in the granules may block adsorption sites also bind the particles, inhibiting the ability to expand and reform LDH layers with the dye molecules in the interlayer gallery. PMMA is also a hydrophobic which will contribute to a reduced interface between the MMO surface and the adsorbate solution.
Continuous synthesis of LDH composites was carried out in a two-stage reactor system. The core@shell composites were designed to combine properties of the core with the adsorptive capacity of LDHs. For this research ZnO was chosen as the core material due to its photocatalytic capabilities, and Mg2Al-CO3 was chosen for the shell, due to the large Ca2Al-NO3 particles. Zn2Al-CO3 would be difficult to distinguish from the ZnO in chemical analysis. The composite exhibited reflections relating to both ZnO and the Mg2Al-CO3 in the diffractogram. The SBET of the composite (75m2 g-1) was greater than each of the individual materials and a dry mixture of LDH and ZnO powders. There is a reduction in Mg/Al ratio of the LDH from 2:1 to closer to 1:1. This was first attributed to the potential for Zn2Al-CO3 LDH production in the second reactor from residual Zn2+ ions or dissolution of ZnO. However, further examination of synthesis in the second reactor indicated that NaOH concentration has a dramatic effect on Mg(OH)2 precipitation. Low NaOH caused Al(OH)3 to be prevalent in the brucite-like sheets. This may phenomenon may occur in composite synthesis. Increased Al(OH)3 distorts the layers and caused an increase in SBET to 250m2 g-1. The synthesis of ZnO@LDH composite materials was partially successful as there appears to be some degree of bonding between ZnO cores and Mg2Al-Co3 LDH shells. However, more work to fine tune continuous composite synthesis is required, as there is in some instances a mixture of LDH and ZnO nanoparticles that are not part of a composite material.
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