Novel Materials for Nitrite Detection and Phosphate Removal

Abstract

This thesis focuses on the development of materials for environmental applications, especially for the detection and removal of nutrients in water sample. There are two researches including the development of colorimetric sensor based on mesoporous silica nanoparticles for nitrite detection and bio–adsorbent based on calcium silicate hydrate nanoparticles composite bio–cryogel for phosphate removal. For the first part of this work, a novel colorimetric sheet based on Griess reagent–doped mesoporous silica nanoparticles was developed for nitrite detection since nitrite is an important indicator of water quality which can accelerate eutrophication and damage ecosystems. It can cause blue baby syndrome in infants and contribute to the formation of carcinogenic N–nitrosamines. Griess reagent was adsorbed on long–range ordered hexagonal mesoporous silica nanoparticles and developed ink–bottle pores with some disorder. When the modified nanoparticles were bound using starch to fabricate a thin (313 µm) colorimetric sheet, spherical particles with a rougher surface and some distortion of their mesoporosity were observed. The sheet was used in conjunction with digital image colorimetry (DIC) to provide a wide linear range (0.05 to 2.5 mgL–1) with a low nitrite detection limit (15 µgL–1–NO2–, equal to 4.5 µgL−1 NO2––N), good inter–day precision (1.93 %RSD), and excellent accuracy (2.67% relative error). The colorimetric sensors produced from the sheet costs only 0.04 USD each, while the DIC uses a standard smartphone for photography. It thus offers an easier and cheaper means of conducting rapid on–site analysis of nitrite in water with reliable quantitative results. For the second part of the thesis, a novel calcium silicate hydrate composite cryogel (Cry–CSH) was successfully prepared for phosphate removal and recovery as phosphate is a major pollutant that deteriorates water quality and causes eutrophication. Calcium silicate hydrate (C–S–H) was mixed with the gel precursor (7.5% w/w) prepared from native starch and limewater (saturated calcium hydroxide solution as the cross–linker). The mixture was frozen and thawed for 3 cycles giving an interconnected macroporous composite. This had C–S–H nanoparticles (75 mg) immobilized on a monolithic floatable cryogel network (2.5 cm diameter×1.0 cm height) enabling an easier recovery and without the losses that occur when using C–S–H nanoparticles. The phosphate adsorption reaches equilibrium at 120 min with adsorption capacity of 2.50 mgPO43-/gCry-CSH (65.42 mgPO43-/gC–S–H) under optimum conditions. Adsorption equilibrium data were well fit by the Freundlich isotherm model, while kinetic results were well fit by the pseudo second–order model. The calculated activation energy (Ea) of 43.9 kJ/mol indicates chemical adsorption, while a positive change in enthalpy (ΔH0, 19.3 kJ/mol) indicates the endothermic nature of phosphate adsorption. Cry–CSH can remove phosphate from wastewater and effluent samples with excellent removal efficiency (> 98%). It can float on water surface for at least 105 days without damage, while its phosphate adsorbed form can be biodegraded within 10 days under soil buried conditions. Both colorimetric sensor and bio–adsorbent developed in this research would be benefit for environmental applications, especially for Phuket that nutrients have increased in aquatic ecosystems due to various anthropogenic activities leading to deterioration of the ecosystem and can accelerate eutrophication.