A. Violante

Effect Of ph, Phosphate And Oxalate On The Adsorption/desorption Of Arsenate On/from Goethite

We studied (i) the competitive adsorption of arsenate and phosphate on goethite as affected by pH, anion concentration, and order of anion addition; (ii) the effect of increasing amounts of arsenate or phosphate on the desorption of phosphate or arsenate from the surfaces of goethite; and (iii) the influence of oxalate on the desorption of arsenate and phosphate from goethite. Similar amounts of phosphate and arsenate were adsorbed on goethite samples at pH 3.0-8.5. By adding arsenate and phosphate as a mixture (As + P systems; arsenate/phosphate molar ratio of 1), the amounts of arsenate and phosphate adsorbed on goethite were similar at pH > 6.0, but at pH < 6.0 slightly more arsenate than phosphate was adsorbed. When arsenate was added before phosphate (As before P systems), the efficiency of arsenate in inhibiting the adsorption of phosphate on goethite was higher than that of phosphate in inhibiting the adsorption of arsenate, when phosphate was added before arsenate (P before As systems). Furthermore, we found that the decrease in adsorption of phosphate in the presence of increasing concentrations of arsenate was greater than that of arsenate in the presence of increasing concentrations of phosphate. There is much evidence indicating that arsenate and phosphate compete primarily for a similar set of surface sites on goethite, but they are adsorbed on some sites uniquely specific for arsenate and phosphate. In the pH range of 3.0 to 8.5, much more phosphate previously adsorbed on goethite was desorbed by arsenate (mainly at pH < 6.0) than arsenate by phosphate. The desorption of both the ligands was influenced by time. Finally, the presence of oxalate ions in solution desorbed higher amounts of phosphate than arsenate ions, adsorbed previously on goethite, mainly in acidic systems.

Adsorption, activity and kinetic properties of urease on montmorillonite, aluminium hydroxide and AL(OH)x-montmorillonite complexes

Abstract The adsorption of urease on a montmorillonite (M), a non-crystalline aluminium hydroxide (AL) and an Al(OH) x -montmorillonite complex (AM) as well as the activity, the kinetics and the stability of the enzyme-clay mineral complexes were studied. The equilibrium adsorption isotherms of urease on clay minerals fitted both the Langmuir or the Freundlich equations. The Langmuir adsorption isotherm of the enzyme on M was of H type (“high affinity”) whereas the isotherms on AL and AM were of L type (“Langmuir”). On adding up to 21.2mg of enzyme g −1 clay, the amount of urease held on the clay minerals followed the order M > AM > AL throughout the pH range explored (4.0–9.0). The adsorption of urease on M, AM and AL was differently affected by pH. The specific activity of enzyme immobilized on M and AM was relatively high (71 and 64% respectively) as compared to that of the free enzyme; in contrast the specific activity of urease adsorbed on AL was considerably reduced (15%). The free and immobilized urease showed similar pH- and temperature-activity profiles and both states obeyed Michaelis-Menten kinetics. The V max and K m parameters, as well as the thermal stability of adsorbed urease were always lower than those of the free urease, whereas the proteolytic stability of urease held on AL was higher than that of the enzyme free or adsorbed on M and AM. Finally, it was ascertained that the covering of the surfaces of montmorillonite with different amounts of OH-A1 species reduced the quantity as well as the activity of adsorbed enzyme.

Invertase β-fructosidase): Effects of montmorillonite, AL-hydroxide and AL(OH)x-montmorillonite complex on activity and kinetic properties

Abstract The effects of a montmorillonite (M), a non-crystalline aluminium hydroxide (AL) and an Al(OH)2-montmorillonite complex (a montmorillonite partially coated with OH-Al ions; AM) on the adsorption, residual activity and kinetic properties of invertase were determined. In the range of pH 3.0–9.0 the ability of the clay minerals to adsorb protein molecules was in the order M > AM > AL and was influenced by the nature of the buffer solution. Adsorption resulted in a significant reduction of invertase activity (more than 85% at pH 4.65). the extent of which depended on clay mineral (AL > AM > M) and on pH. The free and immobilized enzyme showed maximum activity in the range of pH 4.5–5.5 and both states obeyed Michaelis-Menten kinetics. The Vmax, and Km values indicated a partial non-competitive inhibition of the clay minerals. Invertase adsorbed on AM and AL was more stable to thermal deactivation than was free enzyme, whereas invertase held on M was much less stable. In contrast, invertase adsorbed on M was more stable to the action of protease than free enzyme or that adsorbed on AM or AL.

Pesticide effects on the activity of free, immobilized and soil invertase

Abstract The influence of four pesticides (atrazine, carbaryl, glyphosate and paraquat) on the catalytic behaviour of invertase, either free, immobilized on inorganic and organic soil colloids or in soils, was investigated. Invertase was immobilized on a clean clay (montmorillonite), an organic compound (tannic acid), and on synthetic organo-mineral [Al(OH) x -tannate and Al(OH) x -tannate-montmorillonite] complexes. Soils with different physico-chemical properties were utilized. The effects of pesticides on invertase performance depended not only on the nature of the pesticide but also on the “status” of the enzyme, i.e. if free, immobilized or in soil. Glyphosate and paraquat enhanced the activity of invertase either free or immobilized on montmorillonite and both pesticides behaved as mixed-type non-essential activators. Activity decreases were instead measured for the enzyme immobilized on organic and organo-mineral matrices. Contrasting results (increases, decreases and no effects) were detected for soil invertase. A general inhibition effect was exhibited by methanol on free, immobilized or soil invertase, but the extent of inhibition depended on the enzyme microenvironment. The addition of atrazine and carbaryl caused partial increases of free and immobilized invertase activity, whereas carbaryl further reduced enzymatic activity in some soils.

Interactions of invertase with tannic acid, hydroxy-aluminium (OH-A1) species or montmorillonite

Interactions between invertase, tannic acid and hydroxy-aluminium (OH-A1) polymers were investigated with regard to their influence on enzyme activity and the formation of active enzymatic complexes. OH-A1 species slightly inhibited invertase activity (inhibition constant, Ki = 133 him) whereas a marked inhibition (>90%) was detected at a tannic acid concentration greater than 1.25 mM. The Vmax and Km values indicated a pure non-competitive and a mixed-type inhibition mechanism of OH-A1 species and tannic acid, respectively. Depending on the incubation time and the tannic acid-to-enzyme ratio, the interaction between invertase and tannic acid yielded both soluble and insoluble complexes, which displayed reduced activity levels. The decrease of invertase activity was a function of both the tannic acid-to-invertase ratios and the contact time. Bonding between invertase and tannic acid as well as the residual activity of the immobilized enzyme were greatly increased by the presence of OH-A1 polymers during the complexation process. Many more active invertase molecules were removed from solution and an enhanced precipitation of active invertase-tannic acid complexes was observed. Clearly not only OH-A1 ions facilitated flocculation of tannate-invertase complexes with different structural characteristics but their interaction with tannic acid molecules gave rise to Al precipitation products having a different charge and sorption sites. Furthermore, an invertase-OH-Al-tannic acid complex was anchored on montmorillonite surfaces and the complex formed showed a relatively high enzymatic activity. OH-A1 species acting as bridges between tannate molecules and montmorillonite surfaces also facilitated the immobilization of soluble invertase-OH-Al-tannate complexes.

Comparison of adsorption of phosphate, tartrate, and oxalate on hydroxy aluminum montmorillonite complexes

Competitive adsorption between phosphate, tartrate and oxalate was studied on two hydroxy aluminum montmorillonite complexes (AlMt1.6 and AlMt6), which were prepared by adding a base to pH 5.5 to samples containing 1.6 and 6.0 mol Al per kg clay. The quantities of phosphate, tartrate and oxalate adsorbed were more closely related to the amount of OH-Al species coatings on the montmorillonite than to the surface area of the complexes. The adsorption capacity of phosphate was much greater than that of tartrate or oxalate for both samples. Adding molar amounts of oxalate and tartrate resulted in an oxalate/tartrate adsorption ratio (Rf) of ∼1. However, in the presence of phosphate, Rf values were <1.0, and the Rf values decreased with increasing amounts of added phosphate, indicating that tartrate competed with phosphate more effectively than oxalate. The presence of tartrate also reduced phosphate adsorption by the complexes. The efficiency of tartrate in reducing phosphate adsorption increased by increasing the initial tartrate/phosphate molar ratio and by adding tartrate 2 h before phosphate addition. Tartrate and oxalate added as a mixture in equimolar quantities were much more effective in inhibiting phosphate sorption than tartrate alone under the same organic ligand concentrations, probably because more sites with high affinity for both the organic ligands were occupied by tartrate and oxalate than by tartrate alone. The efficiency of tartrate alone, or combined with oxalate, in preventing phosphate adsorption was greater for the complex containing a lesser amount of OH-Al species coating the montmorillonite surfaces. This result may be attributable to a greater proportion of sites specific for organic ligands present on AlMt1.6 compared to AlMt6 complex.

EFFECT OF TANNATE, pH, SAMPLE PREPARATION, AGEING AND TEMPERATURE ON THE FORMATION AND NATURE OF Al OXYHYDROXIDES

The aim of this work was to study the influence of tannate concentration (tannic acid/Al molar ratio [R] of 0.01–0.1), pH (5.0 and 8.0), order of aluminum, hydroxyl, and tannate addition, and prolonged ageing at different temperatures (10 months at 50°C and 5 y at room temperature and further ageing for 3–15 days at 140°C) on the nature and crystallinity of synthetic boehmite. Tannate facilitated the formation of boehmite relative to Al(OH)3 polymorphs when coprecipitated with Al and also when added to Al(OH)x phases already formed at pH 4.5 or 8.0. However, the organic ligand was more effective in preventing or perturbing the growth of the crystals of boehmite when coprecipitated with Al than when added to soluble Al(OH)x species or preformed Al precipitates. Boehmite aged at temperatures ⩽50°C typically showed a fibrous morphology. Crystals with a rhombic habit were observed in some treatments after ageing at 140°C. Significant amounts (15–30% C) of tannate were present in the precipitates after prolonged ageing. This tannate was only partly (<50%) oxidized by repeated treatments with H2O2. With further ageing at 140°C, the crystallite size of selected samples increased only slightly, suggesting that much of the remaining tannate was present in the internal structure of these samples and not simply adsorbed to their surfaces. Atomic force microscopy observation showed the presence of globular nanoparticles (probably Al tannate precipitates) attached to the elongated crystals of boehmite. Taken together, our results demonstrate that the crystalline phases that formed under mild conditions in the presence of low concentrations (R = 0.01) of foreign ligands have the same structure as boehmite, but with a poorly ordered and defective ion arrangement. At higher ligand concentrations (R ⩾0.05), mixtures of materials are formed having varying degrees of order, particle size and morphology.