Temperature swing adsorption of heavy metals on novel phosphate-type adsorbents using thermosensitive gels and/or polymers

A novel thermosensitive adsorbent was developed, which adsorbs and/or desorbs heavy metals through temperature swing process. The gel-type and polymer-type adsorbents, composed of N-isopropylacrylamide (NIPA) as a thermosensitive component and 2-methacryloyloxyethyl phosphate (MEP) as an interactive component, were prepared by free radical copolymerization. For each type of poly(NIPA-co-MEP), phase transitions and temperature dependences for the amount of Cu, a model metal ion, adsorbed was examined. The proposed mechanism associated with the temperature swing adsorption (TSA) of Cu to poly(NIPA-co-MEP) is as follows. In the case of the shrinking gel at higher temperatures, two MEP groups are positioned so as to interact with one Cu ion, while in the swelling gel at lower temperatures, Cu is desorbed from isolated MEP groups. In the case of copolymers, at temperatures lower than the lower critical solution temperature (LCST), two MEP groups interact with one Cu ion as well as those in shrunken gels, and at temperatures higher than the LCST, an aggregate of the copolymer, which is strongly hydrophobic, ejects free water along with Cu ion in the shrinking process. The temperature dependences for adsorption to the copolymer are opposite to gels, even in the gel with a low density of crosslinking points

Dewatering of inorganic sludge using dual ionic thermosensitive polymers

The dewatering of inorganic sludge by a novel method based on the hydrophilic/hydrophobic transition of ionic thermosensitive polymers was investigated. As an inorganic sludge, the drinking water treatment sludge was used. Cationic thermosensitive polymers, poly(NIPAM-co-DMAPAA), which were synthesized by copolymerizing N,N-dimethylaminopropylacrylamide (DMAPAA) as the cationic component with N-isopropylacrylamide (NIPAM) as the thermosensitive component, were effective in dewatering the sludge. When the dewatering was performed at room temperature, the optimum polymer dosage for the dewatering rate was observed similar to the conventional hydrophilic polymeric flocculants. On the other hand, when the dewatering was carried out above the lower critical solution temperature (LCST) of poly(NIPAM-co-DMAPAA), the dewatering rate increased remarkably as the polymer dosage increased. Such high dewatering rates can be attributed to the hydrophobic interaction among the thermosensitive polymer molecules adsorbed on the sludge. However, the LCST of poly(NIPAM-co-DMAPAA) increased considerably as the DMAPAA content increased. In order to solve this problem, the use of dual ionic thermosensitive polymers was investigated. By using the anionic thermosensitive polymer, poly (NIPAM-co-AAC), which was synthesized by copolymerizing acrylic acid (MC) with NIPAM, in combination with cationic poly(NIPAM-co-DMAPAA), the dewatering rate was remarkably increased at relatively low temperatures. This increase was attributed to the formation of a polymer complex that decreased the LCST of the polymer molecules adsorbed on the sludge

Preparation of a novel composition-gradient thermosensitive gel

The feasibility of a novel composition-gradient copolymer gel, in which the composition gradually changes with the distance, was examined. The slab-shaped copolymer gels of a thermosensitive primary component, N-isopropylacrylamide (NIPA) and an ionic secondary component, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) or acrylic acid (AA), were prepared between two substrates of hydrophilic glass and hydrophobic polytetrafluoroethylene (Teflon). In the NIPA-co-AMPS gels, prepared at 40°C between glass and Teflon, the content of AMPS gradually decreased toward Teflon wall. The formation of the gradient composition can be attributed to the repulsion of hydrophilic AMPS to Teflon. The composition-gradient NIPA-co-AMPS gel bended and stretched reversibly without breaking in response to the change in temperature. On the other hand, in the NIPA-co-AA gels prepared at 40°C between glass and Teflon, AA distributed homogeneously. The difference in the distributions of ionic components in NIPA-co-AA and NIPA-co-AMPS gels comes from the differences in the reactivities and interactions of the ionic components with NIPA. The conversion of ionic components and the rate of gelation of NIPA-co-AA are larger than those of NIPA-co-AMPS. The glass transition temperatures of NIPA-co-AA gels are higher than those of NIPA and AA gels. This fact is attributed to that NIPA and AA in the copolymer gels are strongly interacting by hydrogen bonding with amide and carboxylic groups. The essential points to prepare a composition-gradient copolymer gel are that in addition to the repulsion to a substrate the secondary component has little interaction with the primary component

Experimental and theoretical investigation of ligand effects on the synthesis of ZnO nanoparticles

ZnO nanoparticles with highly controllable particle sizes(less than 10 nm) were synthesized using organic capping ligands in Zn(Ac)2 ethanolic solution. The molecular structure of the ligands was found to have significant influence on the particle size. The multi-functional molecule tris(hydroxymethyl)-aminomethane (THMA) favoured smaller particle distributions compared with ligands possessing long hydrocarbon chains that are more frequently employed. The adsorption of capping ligands on ZnnOn crystal nuclei (where n = 4 or 18 molecular clusters of(0001) ZnO surfaces) was modelled by ab initio methods at the density functional theory (DFT) level. For the molecules examined, chemisorption proceeded via the formation of Zn...O, Zn...N, or Zn...S chemical bonds between the ligands and active Zn2+ sites on ZnO surfaces. The DFT results indicated that THMA binds more strongly to the ZnO surface than other ligands, suggesting that this molecule is very effective at stabilizing ZnO nanoparticle surfaces. This study, therefore, provides new insight into the correlation between the molecular structure of capping ligands and the morphology of metal oxide nanostructures formed in their presence

Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification

We investigated the optical properties of colloidal-synthesized ZnO spherical nanoparticles prepared from 1-octadecene (OD), a mixture of trioctylamine (TOA) and OD (1:10), and a mixture of trioctylphosphine oxide (TOPO) and OD (1:12). It is found that the green photoluminescence (PL) of samples from the mixture of TOA/OD and TOPO/OD is largely suppressed compared with that from pure OD. Moreover, it is found that all spherical nanoparticles have positive zeta potential, and spherical nanoparticles from TOA/OD and TOPO/OD have a smaller zeta potential than those from OD. A plausible explanation is that oxygen vacancies, presumably located near the surface, contribute to the green PL, and the introduction of TOA and TOPO will reduce the density of oxygen vacancies near the surfaces. Assuming that the green emission arises due to radiative recombination between deep levels formed by oxygen vacancies and free holes, we estimate the size of optically active spherical nanoparticles from the spectral energy of the green luminescence. The results are in good agreement with results from TEM. Since this method is independent of the degree of confinement, it has a great advantage in providing a simple and practical way to estimate the size of spherical nanoparticles of any size. We would like to point out that this method is only applicable for samples with a small size distribution

Compaction of TiO2 suspension by using dual ionic thermosensitive polymers

The compaction of TiO2 suspension as a negatively charged model suspension by using dual ionic thermosensitive polymers was investigated. First, a cationic thermosensitive polymer, poly(N-isopropylacrylamide-co-N,N-dimethylaminopropylacrylamide) was added to the TiO2 suspension, followed by an anionic thermosensitive polymer, poly(N-isopropyl- acrylamide-co-acrylic acid). By adding the latter, a polymer complex with the cationic thermosensitive polymer adsorbed on the particle is formed, and charge neutralization occurs to decrease the transition temperature of the polymer adsorbed on the TiO2 particles. As a result, the compaction of the TiO2 suspended particles due to the hydrophobic interaction of the adsorbed polymers occurs at a relatively low temperature by applying an adequate mechanical force with a plunger. In order to initiate the compaction, it was necessary to cover the surface of the TiO2 particles sufficiently by the cationic thermosensitive polymer molecules, and the optimum dosage of the anionic thermosensitive polymer was observed. The transition temperature of the polymer complex adsorbed on the TiO2 particles was dependent on the ratio of the dosages of the cationic and anionic thermosensitive polymers