Novel amorphous silica and silica xerogels for the adsorption of organic pollutants from water

This thesis was previously held under moratorium from 9th November 2017 until 9th November 2022.The demand on water supplies is becoming more intensive, thus the need for innovative and cost effective produced water treatment technologies is rising. Produced water is the largest by-product generated by oil and gas extraction, hence, there are significant quantities requiring remediation. Furthermore, over recent years, the presence of other organics known as Endocrine Disrupting Chemicals (EDCs) in various sewage discharges, fresh- and estuarine-waters, has been extensively reported, becoming an increasing source of concern. Membranes are a promising technology for water remediation and the use of an adsorption media as a prior treatment could make them a potentially cost-effective option. Among solid sorbents, hydrophobic aerogels exhibit attractive properties for oil spill applications; however, very few studies have investigated their adsorption performance at low organic concentrations and the cost of functionalisation of these materials, hinder their use as sorbents in the final stages of produced water treatments. This work studies the adsorption of benzene, toluene and the EDC 3,4-dichloroaniline (3,4-DCA) below their solubility limits on hydrophilic and hydrophobic samples of the novel amorphous silica Quartzene and on silica xerogels, synthesised by a low cost and environmentally friendly synthesis, inspired from the work of Bangi, Rao A.P., Hirashima and Rao A.V. [1]. All adsorbents used in this study were also characterised by SEM, FTIR and BET analysis. The full adsorption capacity of hydrophilic Quartzene was achieved in less than 10 h, with 70-90% of uptake in the first 6 h. Adsorption capacities were found to be up to 264 mg/g for benzene and 78.8 mg/g for toluene. Hydrophobic Quartzene, demonstrated an equilibrium value four times higher that of the hydrophilic analogue, reached in less than 3 h at a concentration of 200 ppm benzene. With regard to the synthesised xerogels, evaporation rates and the optimal combination of reactants were identified, in order to obtain the desired degree of hydrophobicity and a surface area mainly due to pores with sufficient dimension to allow pollutant penetration. The so synthesised xerogels adsorb dissolved benzene and 3,4-DCA from water, showing no damage of the granules after days of rotary stirring, suggesting potential re-use. The adsorption behaviour is described by a two-step mechanism, with equilibrium times of ~24 h, with adsorption capacities up to 75 mg/g for benzene and 12.5 g for 3,4-DCA. Micro Column Rapid Breakthrough tests coupled with Sequential Injection Analyses were also conducted, in order to obtain a preliminary evaluation of the adsorbents operative feasibility under conditions more similar to those typically used in water treatment facilities. Micro-columns were designed and 3D printed in methacrylate. Automated solid-phase extraction was used to verify desorption of the analytes by methanol injection. The removal efficiency of the hydrophobic sample of Quartzene was found to be >5.85 mg/g of benzene, with an initial concentration of 73.3 mg/L. The removal efficiency of the silica xerogels was found to be >22.62 mg/g for benzene, with an initial concentration of 105.12 mg/L; the corresponding uptakes of 3,4-DCA were found to be >4.63 mg/g and >7.17 mg/g, respectively, at flowrates of 1.8 mL/min and 0.6 mL/min and with an initial concentration of 16-20 mg/L. Higher removal efficiencies coupled with higher rate of adsorption, which would lead to lower Empty Bed Contact Times (EBCTs), would be required for all the sorbents tested here to be used for treatment of produced water prior to a membrane configuration, especially in offshore facilities. The rate of adsorption of 3,4-DCA, and the related adsorption capacity of the silica xerogels studied here could be promising for large scale application to removal of pesticides and other organic micropollutants, as filling of filters, cartridges or permeable reactive barriers.The demand on water supplies is becoming more intensive, thus the need for innovative and cost effective produced water treatment technologies is rising. Produced water is the largest by-product generated by oil and gas extraction, hence, there are significant quantities requiring remediation. Furthermore, over recent years, the presence of other organics known as Endocrine Disrupting Chemicals (EDCs) in various sewage discharges, fresh- and estuarine-waters, has been extensively reported, becoming an increasing source of concern. Membranes are a promising technology for water remediation and the use of an adsorption media as a prior treatment could make them a potentially cost-effective option. Among solid sorbents, hydrophobic aerogels exhibit attractive properties for oil spill applications; however, very few studies have investigated their adsorption performance at low organic concentrations and the cost of functionalisation of these materials, hinder their use as sorbents in the final stages of produced water treatments. This work studies the adsorption of benzene, toluene and the EDC 3,4-dichloroaniline (3,4-DCA) below their solubility limits on hydrophilic and hydrophobic samples of the novel amorphous silica Quartzene and on silica xerogels, synthesised by a low cost and environmentally friendly synthesis, inspired from the work of Bangi, Rao A.P., Hirashima and Rao A.V. [1]. All adsorbents used in this study were also characterised by SEM, FTIR and BET analysis. The full adsorption capacity of hydrophilic Quartzene was achieved in less than 10 h, with 70-90% of uptake in the first 6 h. Adsorption capacities were found to be up to 264 mg/g for benzene and 78.8 mg/g for toluene. Hydrophobic Quartzene, demonstrated an equilibrium value four times higher that of the hydrophilic analogue, reached in less than 3 h at a concentration of 200 ppm benzene. With regard to the synthesised xerogels, evaporation rates and the optimal combination of reactants were identified, in order to obtain the desired degree of hydrophobicity and a surface area mainly due to pores with sufficient dimension to allow pollutant penetration. The so synthesised xerogels adsorb dissolved benzene and 3,4-DCA from water, showing no damage of the granules after days of rotary stirring, suggesting potential re-use. The adsorption behaviour is described by a two-step mechanism, with equilibrium times of ~24 h, with adsorption capacities up to 75 mg/g for benzene and 12.5 g for 3,4-DCA. Micro Column Rapid Breakthrough tests coupled with Sequential Injection Analyses were also conducted, in order to obtain a preliminary evaluation of the adsorbents operative feasibility under conditions more similar to those typically used in water treatment facilities. Micro-columns were designed and 3D printed in methacrylate. Automated solid-phase extraction was used to verify desorption of the analytes by methanol injection. The removal efficiency of the hydrophobic sample of Quartzene was found to be >5.85 mg/g of benzene, with an initial concentration of 73.3 mg/L. The removal efficiency of the silica xerogels was found to be >22.62 mg/g for benzene, with an initial concentration of 105.12 mg/L; the corresponding uptakes of 3,4-DCA were found to be >4.63 mg/g and >7.17 mg/g, respectively, at flowrates of 1.8 mL/min and 0.6 mL/min and with an initial concentration of 16-20 mg/L. Higher removal efficiencies coupled with higher rate of adsorption, which would lead to lower Empty Bed Contact Times (EBCTs), would be required for all the sorbents tested here to be used for treatment of produced water prior to a membrane configuration, especially in offshore facilities. The rate of adsorption of 3,4-DCA, and the related adsorption capacity of the silica xerogels studied here could be promising for large scale application to removal of pesticides and other organic micropollutants, as filling of filters, cartridges or permeable reactive barriers