Point Of Zero Salt Effect Research Articles

Stability of Cl-, NO3-, ClO4-, and S04-surface complexes at synthetic goethite/aqueous electrolyte interfaces

Abstract We carried out potentiometric titrations of a synthetic goethite in solutions of NaCl, NaNO3, NaC1O4, and Na2SO4 to determine the intrinsic acidity constant and intrinsic complexation constants of Cl-, NO3 -, ClO4 -, and S04 - surface complexes formed at the goethite/aqueous electrolyte interfaces . The determined values for the four electrolytes were in a narrow range of 5.9-6.2. The values of were found to be 6.7, 6.5, and 6.2 for Cl-, NO3 -, ClO4 -, and S04 -, respectively, and took the value of 16.0 under the assumption that one SO4 2- occupies two charged sites. The values of point of zero salt effect (PZSE), which is defined as the pH value at which titration-curves in an electrolyte intersect, were 8.5, 8.6, 8.2, and 8.9 for NaCl, NaNO3, NaC1O4, and Na2SO4, respectively. There was a highly positive correlation between the PZSE and . Selectivity coefficients for stoichiometric Cl-NO3 and NO3-SO4 exchange reactions on goethite were calculated from the values and exchange isotherms were drawn.

Anion Exchange Chemistry of Middle Atlantic Soils: Charge Properties and Nitrate Retention Kinetics

AbstractThe negative impact of nitrate (NO3) on groundwater supplies has sparked a great deal of interest and concern in recent years, particularly in areas where coarse‐textured soils abound. In light of this concern, the anion exchange chemistry of eight soils from the Middle Atlantic region was studied with particular emphasis on NO3 retention and kinetics. The soils were chosen to encompass a range of physicochemical and mineralogical properties and were extensively characterized. Anion exchange capacity (AEC) was determined on Cl‐saturated samples by desorption of Cl with SO4. Anion exchange capacity ranged from 0 to 1.35 cmolc kg−1 for the eight soils and was found to parallel increases in clay and Fe oxide contents in the soil profiles. Point of zero salt effect (PZSE) values were determined by potentiometric titration with 0.001, 0.01, and 0.1 M NaCl as the indifferent electrolyte. These were of little value in predicting the development of AEC for the soils. The kinetics of NO3 adsorption and desorption were studied using a stirred‐flow reaction chamber and a first‐order reaction best described the data. Nitrate adsorption was found to be completely reversible, indicating a simple electrostatic retention mechanism. The effect of pH and NO3 concentration on cumulative NO3 adsorbed (CNA) and on NO3 adsorption kinetics was also studied. The CNA was found to increase with a decrease in pH below 5.5 and to increase with increasing NO3 concentration. The latter indicated an increase in competitiveness by NO3 for positively charged sites.

Two‐site Model for Aluminum Oxide with Mass Balanced Competitive pH + Salt/Salt Dependent Reactions

AbstractA backtitration technique was used to collect proton isotherm data for an Al oxide, and was found to be stoichiometrically related to the cation and anion isotherm behavior. The adsorption of various ions by the Al oxide surface was modeled based on: (i) two surface sites, (ii) pH + salt‐dependent reactions (H+ and Cl‐, 2OH‐, or Na+ and OH‐), (iii) competitive salt‐dependent reactions (Na+ or Cl‐), (iv) CO2(aq) pH‐dependent reactions with the surface‐OH groups, and (v) presence of an unknown M+Cl‐ salt (0.0005 M). The total number of Al sites was 1.7 µmol m−2 or 3.4 µmol m−2 of total available sites. The pH of point of zero salt effect (PZSE) represented the surface condition in which the negative charges (or cation surfaces) equaled the positive charges (or anion surfaces); the cation exchange capacity (CEC) equaled the anion exchange capacity (AEC) at this value. The CEC‐AEC data and proton isotherm data were stoichiometrically correlated. The pH of PZSE values ranged from 7.50 to 7.76 depending on the electrolyte concentration present, with lower values as the concentration increased. The negative shifts in the PZSE values are due to anion impurities initially present on the surface and positive shifts are due to cation impurities.

Salt effect in a multicomponent variable charge system: curve of zero salt effect, registered in a pH‐stat

SUMMARYMeasurement of the effect of KC1 addition to a soil suspension in pH‐stat experiments indicates that the pH titration curves of such a multicomponent system are characterized by a Curve of Zero Salt Effect (CZSE) rather than a Point of Zero Salt Effect (PZSE). This curve is most accurately measured as a functional relation between pH and KC1 molarity. The described method is very accurate and, by its rapidity, precludes long‐term changes in the sample which are the bane of classic batch experiments. In analogy, multicomponent soil systems will probably be characterized by a Curve rather than a Point of Zero Net Charge (PZNC).

Backtitration Technique for Proton Isotherm Modeling of Oxide Surfaces

AbstractBatch potentiometric titration analyses were made on a γ‐Al2O3 colloidal suspension and interpreted in terms of the electroneutrality principle. Three titration methods involving a singular reference curve were compared. The first reference used was theoretical, the latter two were representative supernatant curves: an electrolyte solution, and an aliquot of the zero point of titration (ZPT) supernatant. All three methods resulted in similar charge isotherms and a zero point of charge (ZPC) = 8.60. A fourth titration method was developed involving each supernatant of the batch titration sample to act as a unique reference. This latter method is a backtitration technique which accounts for all sources that also consume H+ ions, leaving the difference to be the adsorbed proton concentration only. The NaClO4 electrolyte solution used is postulated to be adsorbed with competitive cation and anion exchange mechanisms with a point of zero salt effect (PZSE) = 7.50.

Ionic strength effects on surface charge and adsorption of phosphate and sulphate by soils

SUMMARYTwo surface soils (Patua and Tokomaru) of contrasting mineralogy were incubated with several levels of either CaCO3 or HC1. The effects of ionic strength on pH, on surface charge, and on the adsorption of phosphate and sulphate were measured in three concentrations of NaCl.The pH at which the net surface charge was zero (point of net zero charge—PZC) was 1.8 for the Tokomaru soil and 4.6 for the Patua soil: differences that can be related to mineralogical composition. There was an analogous point of zero salt effect (PZSE) that occurred at pH 2.8 for the Tokomaru soil and at 4.6 for the Patua soil. The presence of permanent negative charge in the Tokomaru soil resulted in an increase in PZSE over PZC.The effect of ionic strength on adsorption varied greatly between phosphate and sulphate. For phosphate, there was a characteristic pH above which increasing ionic strength increased adsorption and below which the reverse occurred. This pH (PZSE for adsorption) was higher than the PZC of the soil and was 4.1 for the Tokomaru soil and 5.3 for the Patua soil. In contrast, increasing ionic strength always decreased sulphate adsorption and the adsorption curves obtained in solutions of different ionic strengths converged above pH 7.0. If increasing ionic strength decreases adsorption, the potential in the plane of adsorption must be positive. Also, if increasing ionic strength increases adsorption, the potential must be negative. This suggests that, depending upon pH, phosphate is adsorbed when the potential in the plane of adsorption is either positive or negative, whereas sulphate is absorbed only when the potential is positive.