Cation-Exchange Properties of Aluminium(III) Phosphate

Potentiometric titrations of aluminium(III) phosphate have been performed in the presence of aqueous electrolyte solutions containing Cu2+, Zn2+, Ni2+ and K+ ions as a function of the concentration of the latter, the pH and the temperature. The sorption of Zn2+ ions has also been studied and the data for metal ion sorption shown to correlate with the Potentiometric titration data, indicating that the process responsible for metal ion uptake is ion exchange. The pKa value of the exchanger and its thermodynamic parameters have been evaluated.

Exchange of Some Divalent Metal Cations and Their Effect on the Dissociation of Iron(III) Phosphate

Potentiometric titrations of iron(III) phosphate have shown an apparently weak monobasic acid behaviour towards alkali, alkaline earth and divalent transition metal ions. The selectivity sequence and dissociation of the exchanger was found to increase in the order Cu2+ > Zn2+ > Ni2+ > Ca2+ > K+. The dissociation constants of iron(III) phosphate have been determined from Potentiometric titration data in the temperature range 303–323 K. In addition, the exchange of Cu2+, Zn2+ and Co2+ on iron(III) phosphate and the effect of these ions on dissociation were also studied as a function of concentration, temperature and solution pH value. The thermodynamic parameters ΔH0, ΔS0 and ΔG0 for the dissociation of iron(III) phosphate are also presented.

Temperature and pH Effect on the Sorption of Divalent Metal Ions by Silica Gel

The sorption of Zn2+, Ni2+ and Cd2+ ions by silica gel was studied as a function of ion concentration, pH and temperature. An increase in all three parameters led to an increase in the extent of sorption for all the metal cations studied. The selectivity of the solid was observed to be in the order Zn2+ > Ni2+ > Cd2+. Sorption of the metal cations was accompanied by the release of H+ ions into the bulk phase. On average two moles of H+ ions were released per mole of metal cation sorbed. The sorption data fitted the linear forms of both the Kurbatov and Langmuir adsorption equations. The values of the binding constants were used to estimate the apparent thermodynamic parameters, ΔH and ΔS, for the adsorption process. FT-IR spectroscopic studies also showed that the uptake of metal cations by the silica gel occurred via a cation-exchange process.

Phosphate/sulphate exchange studies on Amberlite IRA-400

The sorption behaviour of phosphate on strong basic anion exchanger, Amberlite IRA-400 (SO4(2-) form) is studied as a function of pH (3-11) at two different initial concentration ranges (0.161 - 0.807 mmol l(-1), 3.874 - 6.134 mmol l(-1)) at 25 degrees C. The stiochiometry of the exchange reactions is observed to be dependent upon concentration and pH of the solutions. It is suggested that the sorption of phosphate takes place initially as PO4(3-) followed by HPO4(2-)/H2PO4(-). The data obtained is explained with the help of a modified Langmuir and mass law action equations.

Temperature effect on phosphate sorption by iron hydroxide

The ion exchange sorption of phosphate on Fe(OH)3 is studied as a function of temperature (25-55 degrees C) and concentration (1.49-3.23 mmol l(-1)). The mechanism of sorption is observed to be the exchange of OH- anions from the adsorbent surface by those of H2PO4- and HPO4(2-) from the aqueous solution. An equation developed in the Langmuir formalism is used to explain the ion exchange sorption of phosphate on Fe(OH)3. The isosteric heat of sorption illustrates that the uptake of phosphate on Fe(OH)3 takes place through an anion exchange process, involving both the phosphate anions H2PO4- and HPO4(2-). Further, at higher temperature the solid prefers the HPO4(2-) anions forming a binuclear bridging complex.

Selective removal of Pb2+ by AlPO4

Aluminium(III) phosphate was observed to have a very high selectivity towards Pb2+ ions. Under similar conditions, the sorption capacities of the exchanger were observed to follow the order: Pb2+ > Zn2+ > Ni2+ > Co2+. The potentiometric titration data were used to determine the deprotonation constants (pKa) of AlPO4, which were dependent upon the temperature and nature of the metal cations present in the aqueous phase. Different physical methods i.e. FTIR, EPM and the wet chemical analyses were used to investigate the uptake mechanism of Pb2+ by the solid, which showed that no new solid phases were present in the solid residue after sorption of Pb2+ at pH 5. The exchange between protons from the surfaceand Pb2+ from solutions was found to be responsible for Pb2+ sorption by aluminum(III) phosphate.

The Sorption of Divalent Metal Ions on AlPO4

The sorption of Cu(2+), Pb(2+), Ni(2+), and Cd(2+) ions on the aluminum(III) phosphate was observed to increase with increases in the concentration, temperature, and pH of the system. The apparent dissociation (pK(a)), binding (pK(b)) and exchange (pK(ex)) constants of aluminum(III) phosphate were evaluated and found to be dependent upon the temperature and nature of the metal cations. The values of the dissociation constants (pK(a)) followed the order Pb(2+)<Cu(2+)<Cd(2+)<Ni(2+)<K(+), opposite to the values of the binding constants and selectivity shown by the solid. Ion exchange between protons from the surface and metal cations from aqueous solution was found to be responsible for the metal ions sorption by AlPO(4). The values of the dissociation (pK(a)) and exchange (pK(ex)) constants were used to estimate the corresponding apparent thermodynamic parameters (DeltaH(o)(diss), DeltaS(o)(diss) and DeltaH(o)(exch), DeltaS(o)(exch)). The values of DeltaH(o)(diss) and DeltaH(o)(exch) were found to be positive, which showed that both the dissociation and exchange processes were endothermic in nature. The DeltaS(o)(diss) and DeltaS(o)(exch) were also found to be positive, which showed that both the deprotonation and exchange processes on the AlPO(4) were entropy driven and spontaneous.

Selective removal of chromates by macroporous exchanger Amberlyst A-21

Sorption studies of chromate on weakly basic macroporous anion exchanger Amberlyst A-21 were performed as a function of pH (2 - 9), temperature (20 - 40 degrees C) and concentration (0.4 - 80 mmol.l(-1)). Chromate sorption was observed to decrease with the increase in initial pH of the solution, with a sorption maxima at pH 2 and increased with the increase in temperature. The changes in pH and in the ratio of choride ions released to chromate ions sorbed suggested that the anion exchange process was accomanied by hydrolysis and Donnan invasion. The FTIR spectra and isoeteric heat of sorption besides showing the hydrolysis / Donnan invasion of resin under different experimental conditions of concentration, pH and temperature also showed the dimerization of the chromate species inside the resin phase.

Comparative Sorption Properties of Metal(III) Phosphates

Sorptionbehavior of metal(III) phosphates toward Zn(2+) was investigated using different concentrations, pHs, and temperatures. Ion exchange between protons from the surface and metal cations from solutions was found to be responsible for metal sorption by metal(III) phosphates. Dissociation constants of metal(III) phosphates and binding constants of the Zn(2+) with metal(III) phosphates were determined using Henderson-Hasselbach and modified Langmuir equations, respectively. The sorption process in all the three metal phosphates was found to be endothermic in nature while the dissociation process was observed to be endothermic in AlPO(4) and FePO(4) and exothermic in CrPO(4). Copyright 1999 Academic Press.