Kinetics and equilibrium adsorption study of selenium oxyanions onto Al/Si and Fe/Si coprecipitates.

Inappropriate treatments for the effluents from semiconductor plants might cause the releases and wide distributions of selenium (Se) into the ecosystems. In this study, Al/Si and Fe/Si coprecipitates were selected as model adsorbents as they often formed during the wastewater coagulation process, and the removal efficiency of selenite (SeO3) and selenate (SeO4) onto the coprecipitates were systematically examined. The removal efficiency of SeO3 and SeO4 was highly related to surface properties of Al/Si and Fe/Si coprecipitates. The surface-attached Al shell of Al/Si coprecipitates shielded a portion of negative charges from the core SiO2, resulting in a higher point of zero charge than that of Fe/Si coprecipitates. Thus, adsorption of SeO3/SeO4 was favorable on the Al/Si coprecipitates. Adsorptions of both SeO3 and SeO4 on Al/Si coprecipitates were exothermic reactions. On Fe/Si coprecipitates, while SeO3 adsorption also showed the exothermic behavior, SeO4 adsorption occurred as an endothermic reaction. The kinetic adsorption data of SeO3/SeO4 on Al/Si and Fe/Si coprecipitates were described well by the pseudo-second-order kinetic model. SeO4 and SeO3 adsorption on Fe/Si or Al/Si were greatly inhibited by the strong PO4 ligand, whereas the weak ligand such as SO4 only significantly affected SeO4 adsorption. The weakest complex between SeO4 and Al was implied by the essentially SeO4 desorption as SeO4/PO4 molar ratios decreased from 0.5 to 0.2. These results were further confirmed by the less SeO4 desorption (41%) from Fe/Si coprecipitates than that from Al/Si coprecipitates (78%) while PO4 was added sequentially.

Pyrolysis of biomass by thermal analysis-mass spectrometry (TA-MS).

The kinetic parameters such as pre-exponential factor and activation energy of hemicellulose, cellulose, and lignin were well determined by the linear regressions of selected, sufficient thermogravimetric data, and close to literature values. The pyrolysis of biomass can be divided into four stages. There was only drying in the zeroth stage (<150°C). In the first stage (150-250°C), some light hydrocarbons were produced with the early pyrolysis of biomass. The biomass was mainly pyrolyzed in the second stage (250-500°C) with higher reaction rates than those of other stages. The productions of H(2) and CO(2) in the third stage (>500°C) may be able to be the evidence of self-gasification of char existing at higher temperatures.

A geometric approach to determine adsorption and desorption kinetic constants.

A geometric method based on Langmuir kinetics has been derived to determine adsorption and desorption kinetic constants. In the conventional procedure, either the adsorption kinetic constant (k(a)c) or desorption kinetic constant (k(d)c) is found from kinetic experiments and the other is calculated by their correlation with the equilibrium constant, i.e, k(d)c = Kcon/k(a)c, where Kcon has been known from equilibrium studies. The determined constants (Kcon, k(a)c, k(d)c), if based only on the conventional procedure, may not be accurate due to their mathematical dependence. Therefore, the objectives of this study are applying a geometric approach to directly determine Langmuir kinetic constants and describe adsorption behavior. In this approach, both adsorption kinetic constant (k(a)g) and desorption kinetic constant (k(d)g) are obtained only from data of kinetic experiments, and a geometric equilibrium constant (Kgeo) is calculated by Kgeo = k(a)g/k(d)g. The deviation between Kgeo and Kcon can prove the accuracy of k(a)g and k(d)g which were determined by this method. This approach was applicable to selenate, selenite and Mg2+ adsorption onto SiO2 regardless of whether the adsorbate formed inner- or outer-sphere complexes. However, this method showed some deviation between Kcon and Kgeo for Mn2+ adsorption because of the formation of surface Mn(II)-hydroxide clusters, which was inconsistent with the basic assumption of this method of monolayer adsorption.