Alex C.K. Yip

Soil stabilisation using AMD sludge, compost and lignite: TCLP leachability and continuous acid leaching

Utilising locally available industrial by-products for in situ metal stabilisation presents a low-cost remediation approach for contaminated soil. This study explored the potential use of inorganic (acid mine drainage (AMD) sludge and zero-valent iron) and carbonaceous materials (green waste compost, manure compost, and lignite) for minimising the environmental risks of As and Cu at a timber treatment site. After 9-month soil incubation, significant sequestration of As and Cu in soil solution was accomplished by AMD sludge, on which adsorption and co-precipitation could take place. The efficacy of AMD sludge was comparable to that of zero-valent iron. There was marginal benefit of adding carbonaceous materials. However, in a moderately aggressive environment (Toxicity Characteristic Leaching Procedure), AMD sludge only suppressed the leachability of As but not Cu. Therefore, the provision of compost and lignite augmented the simultaneous reduction of Cu leachability, probably via surface complexation with oxygen-containing functional groups. Under continuous acid leaching in column experiments, combined application of AMD sludge with compost proved more effective than AMD sludge with lignite. This was possibly attributed to the larger amount of dissolved organic matter with aromatic moieties from lignite, which may enhance Cu and As mobility. Nevertheless, care should be taken to mitigate ecological impact associated with short-term substantial Ca release and continuous release of Al at a moderate level under acid leaching. This study also articulated the engineering implications and provided recommendations for field deployment, material processing, and assessment framework to ensure an environmentally sound application of reactive materials.

Selective conversion of cellulose into bulk chemicals over Brønsted acid-promoted ruthenium catalyst: one-pot vs. sequential process

Acid hydrolysis and hydrogenation/hydrogenolysis reactions can be combined for catalytic conversion of cellulose into renewable biorefinery feedstocks by using two heterogeneous catalysts: sulfonic acid (–SO3H) functionalized mesoporous silica (MCM-41) and Ru/C. The combination is suitable for a one-pot tandem process to convert cellulose into alkanediols (mainly propylene glycol and ethylene glycol), yet deactivation of the sulfonic acid (–SO3H) functionalized mesoporous silica occurred rapidly after only one reaction cycle because of an irreversible change in the mesoporous structure and loss of acid groups. However, much better selectivity of hexitol or γ-valerolactone (GVL) can be obtained in a sequential tandem process by hydrogenating the hydrolysis products, glucose and levulinic acid (LA). A similar irreversible deactivation of acid catalyst also occurred when it involved the hydrogenolysis of glucose into alkanediols. When the sulfonic acid-functionalized mesoporous silica is filtered, and the hydrolysis products of cellulose are directly used in the hydrogenation reaction without further purification, a better selectivity and stability of hexitol production can be obtained. Under such conditions, the lifetime of the catalyst system can be significantly extended, up to 6 times the original durability of the acid-functionalized silica.

Thermal structural transitions and carbon dioxide adsorption properties of zeolitic imidazolate framework-7 (ZIF-7).

As a subset of the metal-organic frameworks, zeolitic imidazolate frameworks (ZIFs) have potential use in practical separations as a result of flexible yet reliable control over their pore sizes along with their chemical and thermal stabilities. Among many ZIF materials, we explored the effect of thermal treatments on the ZIF-7 structure, known for its promising characteristics toward H2 separations; the pore sizes of ZIF-7 (0.29 nm) are desirable for molecular sieving, favoring H2 (0.289 nm) over CO2 (0.33 nm). Although thermogravimetric analysis indicated that ZIF-7 is thermally stabile up to ~400 °C, the structural transition of ZIF-7 to an intermediate phase (as indicated by X-ray analysis) was observed under air as guest molecules were removed. The transition was further continued at higher temperatures, eventually leading toward the zinc oxide phase. Three types of ZIF-7 with differing shapes and sizes (~100 nm spherical, ~400 nm rhombic-dodecahedral, and ~1300 nm rod-shaped) were employed to elucidate (1) thermal structural transitions while considering kinetically relevant processes and (2) discrepancies in the N2 physisorption and CO2 adsorption isotherms. The largest rod-shaped ZIF-7 particles showed a delayed thermal structural transition toward the stable zinc oxide phase. The CO2 adsorption behaviors of the three ZIF-7s, despite their identical crystal structures, suggested minute differences in the pore structures; in particular, the smaller spherical ZIF-7 particles provided reversible CO2 adsorption isotherms at ~30-75 °C, a typical temperature range of flue gases from coal-fired power plants, in contrast to the larger rhombic-dodecahedral and rod-shaped ZIF-7 particles, which exhibited hysteretic CO2 adsorption/desorption behavior.

Arsenic and copper stabilisation in a contaminated soil by coal fly ash and green waste compost

In situ metal stabilisation by amendments has been demonstrated as an appealing low-cost remediation strategy for contaminated soil. This study investigated the short-term leaching behaviour and long-term stability of As and Cu in soil amended with coal fly ash and/or green waste compost. Locally abundant inorganic (limestone and bentonite) and carbonaceous (lignite) resources were also studied for comparison. Column leaching experiments revealed that coal fly ash outperformed limestone and bentonite amendments for As stabilisation. It also maintained the As stability under continuous leaching of acidic solution, which was potentially attributed to high-affinity adsorption, co-precipitation, and pozzolanic reaction of coal fly ash. However, Cu leaching in the column experiments could not be mitigated by any of these inorganic amendments, suggesting the need for co-addition of carbonaceous materials that provides strong chelation with oxygen-containing functional groups for Cu stabilisation. Green waste compost suppressed the Cu leaching more effectively than lignite due to the difference in chemical composition and dissolved organic matter. After 9-month soil incubation, coal fly ash was able to minimise the concentrations of As and Cu in the soil solution without the addition of carbonaceous materials. Nevertheless, leachability tests suggested that the provision of green waste compost and lignite augmented the simultaneous reduction of As and Cu leachability in a fairly aggressive leaching environment. These results highlight the importance of assessing stability and remobilisation of sequestered metals under varying environmental conditions for ensuring a plausible and enduring soil stabilisation.

Valorization of food waste into hydroxymethylfurfural: Dual role of metal ions in successive conversion steps.

This study aimed to transform food waste into a value-added chemical, hydroxymethylfurfural (HMF), and unravel the tangled effects induced by the metal catalysts on each single step of the successive conversion pathway. The results showed that using cooked rice and bread crust as surrogates of starch-rich food waste, yields of 8.1-9.5% HMF and 44.2-64.8% glucose were achieved over SnCl4 catalyst. Protons released from metal hydrolysis and acidic by-products rendered Brønsted acidity to catalyze fructose dehydration and hydrolysis of glycosidic bond. Lewis acid site of metals could facilitate both fructose dehydration and glucose isomerization via promoting the rate-limiting internal hydride shift, with the catalytic activity determined by its electronegativity, electron configuration, and charge density. Lewis acid site of a higher valence also enhanced hydrolysis of polysaccharide. However, the metals also catalyzed undesirable polymerization possibly by polarizing the carbonyl groups of sugars and derivatives, which should be minimized by process optimization.

Catalytic valorization of starch-rich food waste into hydroxymethylfurfural (HMF): Controlling relative kinetics for high productivity

This study aimed to maximize the valorization of bread waste, a typical food waste stream, into hydroxymethylfurfural (HMF) by improving our kinetic understanding. The highest HMF yield (30mol%) was achieved using SnCl4 as catalyst, which offered strong derived Brønsted acidity and moderate Lewis acidity. We evaluated the kinetic balance between these acidities to facilitate faster desirable reactions (i.e., hydrolysis, isomerization, and dehydration) relative to undesirable reactions (i.e., rehydration and polymerization). Such catalyst selectivity of SnCl4, AlCl3, and FeCl3 was critical in maximizing HMF yield. Higher temperature made marginal advancement by accelerating the undesirable reactions to a similar extent as the desirable pathways. The polymerization-induced metal-impregnated high-porosity carbon was a possible precursor of biochar-based catalyst, further driving up the economic potential. Preliminary economic analysis indicated a net gain of USD 43-236 per kilogram bread waste considering the thermochemical-conversion cost and chemical-trading revenue.

Highly effective degradation of sodium dodecylbenzene sulphonate and synthetic greywater by Fenton-like reaction over zerovalent iron-based catalyst

There is an increasing interest to recycle greywater for meeting non-portable water demand. However, linear alkylbenzene sulphonates (a form of anionic surfactants) that are commonly found in greywater are less biodegradable at moderate to high concentrations. A fenton-like system is a relatively economic advanced oxidation process that can potentially be used for surfactant degradation in greywater treatment. This study investigated the feasibility of zerovalent iron (ZVI)-mediated Fentons oxidation of sodium dodecylbenzene sulphonate (SDBS) using Fe0/H2O2 and Fe2+/Fe0/H2O2 systems under a range of operating conditions. For the Fe0/H2O2 binary system, the initial pH value and Fe0 dosage played important roles in final degradation efficiency. For the Fe2+/Fe0/H2O2 ternary systems, a small amount of Fe2+ (0.5–1.7 mM) contributed a synergistic effect to promote iron recycling and SDBS degradation. Approximately, 90% of SDBS mineralization efficiency was accomplished within 15 min at a pH range from 3.0 to 6.5, using 18 mM Fe0 and 15 mM H2O2. However, the removal kinetics was rate-limited by Fe2+ dissolution from the ZVI surfaces. The Fenton-like process of the Fe2+/Fe0/H2O2 ternary system also presents a promising treatment method for synthetic greywater, in which 90% TOC removal was achieved within the first 10 min; 78% COD and 91% BOD5 were achieved after 120 min of reaction.

Propylene carbonate and γ-valerolactone as green solvents enhance Sn(IV)-catalysed hydroxymethylfurfural (HMF) production from bread waste

Two green solvents, namely propylene carbonate (PC) and γ-valerolactone (GVL), were examined as co-solvents in the conversion of bread waste to hydroxymethylfurfural (HMF) over SnCl4 as the catalyst under microwave heating at 120 °C, and their performances were compared with water and acetone as a common solvent. The results showed that a HMF yield of ∼20 mol% was achieved at 7.5 and 20 min in the PC/H2O and GVL/H2O (1 : 1 v/v) systems, respectively, implying that the tandem reactions (starch hydrolysis, glucose isomerisation, and fructose dehydration) were efficient. The green systems played a critical role in maintaining effective Lewis acid sites, i.e., Sn4+, to a greater extent compared with acetone/H2O and water, where loss of Sn4+ from the liquid phase to colloidal SnO2 particles via hydrolysis was evidenced by X-ray diffraction analysis. In comparison, the utilisation of glucose (47–59 mol% from bread starch within 10 min) appeared as the rate-limiting step in the acetone/H2O and water systems. When comparing PC/H2O with GVL/H2O, the kinetics of overall conversion in the former was more favourable, which was associated with the high in-vessel pressure developed via liberation of CO2 from PC. In addition, dipole moment and dielectric constant of the solvents may also account for their respective performance. This study elucidates the multiple roles of the solvents and their interplay with the catalysts, and advocates the application of green solvents to facilitate catalytic conversion of biomass with a lower energy requirement.

An oriented, siliceous deca-dodecasil 3R (DDR) zeolite film for effective carbon capture: insight into its hydrophobic effect

An all-silica deca-dodecasil 3R (Si-DDR) zeolite with a pore size of 0.36 × 0.44 nm2 is highly desirable for membrane-based separation of CO2 (0.33 nm) from N2 (0.364 nm), which is critical in the post-combustion carbon capture process, via molecular recognition of their slight size difference. For the first time, we acquired h0h-oriented, hydrophobic DDR zeolite films through the epitaxial growth of a DDR seed layer with a structure directing agent of methyltropinium iodide. The degree of the out-of-plane orientation and inter-growth was increased with the secondary growth time, while reducing the defects that provide non-selective pathways. The resulting DDR membrane showed a CO2/N2 separation factor (SF) as high as 11.9 at 50 °C (a representative flue-gas temperature) under dry conditions. More desirably, it could achieve a much enhanced CO2/N2 SF of up to 15.9 at 50 °C in the presence of H2O vapor (3rd largest component in the flue-gas). The transport of the larger N2 molecule, plausibly its entering the pore mouth of DDR zeolites, was more inhibited by H2O molecules adsorbed on the membrane surface; it appears that this surface resistance was due to the hydrophobicity of the highly siliceous DDR membrane and beneficial for improving CO2/N2 SFs under wet conditions.

A nano-sized catalytic architecture composed of SiO/sub 2/-TiO/sub 2/ particle and carbon nanofibers

Amorphous SiO2-TiO2 particles have been grown on the carbon nanofibers successfully via hydrothermal synthesis. SEM and TEM images revealed that the produced particles were in nano size (under 10 nm) and appeared to be distributed uniformly along the curvature of the nanofibers. This work also reveals the possibility of obtaining crystalline particles from amorphous phase via recrystallization. The synthesized composite material could be a potential candidate for catalytic reactions which utilize Ti centre as an active site and which require the particle size of the catalyst in nanometer scale.