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 (A1Mtl.6 and AIMt6), which were prepared by adding a base to pH 5.5 to samples containing 1.6 and 6.0 mol AI per kg clay. The quantities of phosphate, tartrate and oxalate adsorbed were more closely related to the amount of OH-AI 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 irdqibiting 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-A1 species coating the montmorillonite surfaces. This result may be attributable to a greater proportion of sites specific for organic ligands present on A1Mtl.6 compared to A1Mt6 complex. Key Words~Competit ive Adsorption, Hydroxyaluminum-Montmorillonite Complexes, MontmoritlonJte, Oxalate, Phosphate, Tartrate. I N T R O D U C T I O N Relat ively large amounts o f carbon assimilated in photosynthesis are exuded by plant roots. This carbon contains a significant fraction of low-molecular -weight organic acids, such as oxalic, citric, tartaric, mal ic and a series of amino acids (Rovira, 1969; Huang and Violante, 1986; Marschner, 1995). The concentrat ion of organic acids in the soil solution is usually low (10 -3 to 4 X 10 -4 mol 1 ~), but greater amounts are found in the rhizosphere of crop plants (Vance et al., 1996). The area immedia te ly surrounding a root is also a zone of intense biological activity, where most types of microorganisms thrive. The microorganisms in the rhizosphere are more act ive than are those located in bulk soil and synthesize many aliphatic organic acids and phenols. The first studies on the influence o f low-molecularweight organic acids on the sorption and replacement of phosphate by clay minerals started years ago (Swenson et aL, 1949). Because hydrous oxides o f a luminum and iron are largely responsible for the fixation of phosphate in soils, most of the investigations of the effect of organic l igands on the removal and sorption of phosphate were per formed using A1 or Fe oxides (Swenson et al., 1949; Nagarajah et al., 1970; t Permanent address: Department of Soil and Agrochemistry, Huazhong Agricultural University, Wuhan 430070, China_ Hingston et al., 1971; Earl et al., 1979; Yuan, 1980; Sibanda and Young, 1986; Violante et al., 1991, 1996). The results of these studies indicated that the most commonly occurring organic acids in soil, triand dicarboxylic acids were effect ive in reducing phosphate sorption, whereas monocarboxyl ic acids had little effect on phosphate fixation. Hydroxy-A1 interlayered smectites or vermiculi tes are widely distributed in several soil orders. Chemica l interactions be tween plant roots and rhizosphere minerals include formation of noncrystal l ine A1 precipitation products, which may coat the surfaces of phyllosilicates. The presence of OH-A1 species on the surfaces of phyllosi l icates significantly enhances the adsorption of anions on clay minerals (Barnhisel and Bertsch, 1989). Recently, Violante and Gianfreda (1993, 1995) studied the sorption o f phosphate and oxalate and the compet i t ion in sorption between phosphate and oxalate on a hydroxy-A1 montmori l loni te complex. They reported that more phosphate than oxalate was adsorbed, even when the initial amount of oxalate was three t imes greater than that of phosphate. The order of addition of phosphate and oxalate strongly influenced the sorpfion of these ligands, and the max imum reduction in phosphate sorption occurred in acidic systems when oxalate was introduced before phosphate. Unfortunately, in spite of the fact that tarIrate was identified in root exudates and aqueous extracts of forest litter, and is produced by bacteria in Copyright 9 1999, The Clay Minerals Society 226 Vol. 47, No. 2, 1999 Comparison of adsorption on montmorillonite complexes 227 the rhizosphere (Martell and Smith, 1977; Huang and Violante, 1986; Vance et al., 1996), the competitive sorption of tartrate and phosphate on clay minerals and soils has received scant attention (Earl et al., 1979; Violante and Gianfreda, 1995). In the rhizosphere, many different organic ligands may interact with nutrients for adsorbing sites on clay minerals. However, there is no information about the competitive sorption behavior of phosphate (or other nutrients) and organic ligands on clay minerals or soils when two or more organic acids are introduced to the system. This paper describes an investigation of the sorption of phosphate, tartrate, or oxalate and the competitive sorption between phosphate and tartrate, oxalate and tartrate, and among phosphate, tartrate and oxalate on two hydroxy-Al-montmorillonite complexes containing different amounts of OH-A1 species adsorbed on the surfaces of the montmorillonite. MATERIALS AND METHODS Preparation of Al( OH)~-montmorillonite complexes The <2 ixm fraction of Na-montmorillonite from Crook County, Wyoming was separated by sedimentation after ultrasonic dispersion in water. The Al(OH)x-montmorillonite (chlorite-like) complexes (A1Mtl.6 and A1Mt6) were prepared by adding 0.5 mol 1-1 NaOH at the rate of 1 ml min -1 to Na-montmorillonite suspensions containing 1.6 and 6.0 tool A1C13 per kg clay, respectively, until a pH of 5.5 was reached (Barnhisel and Bertsch, 1989; Violante and Gianfreda, 1993). The suspensions were thoroughly stirred during the titration. The final suspensions (2 1) were aged for 24 h and then centrifuged at 10,000 g for 30 rain. The supernatants were filtered through Millipore M.E filters (pore size <0.2 pom, Millipore Co., Bedford, Massachusetts) and used for A1 determination. The sedimented pellets were washed two times with water, dialysed until C1free, and then freeze-dried. The samples were lightly ground to pass a 0.25 mm sieve. The specific surface area was determined gravimetrically using the retention of ethylene glycol monoethyl ether (EGME) (Eltantawy and Arnold, 1973). Exchangeable A1 was extracted using 1.0 mol 1-1 KC1 and determined as described below. The X-ray diffraction patterns of the oriented K-saturated minerals were obtained with a Rigaku diffractometer (Rigaku Co., Tokyo) with Ni-filtered CoKc~ radiation generated at 40 kV and 30 nlA. Adsorption isotherms of phosphate, tartrate and oxalate One hundred milligrams of the complexes were shaken for 24 h at 20~ with a series of 25 ml of 0.02 mol 1 1 KC1 solutions containing different amounts (03.2 mmol 1 1) of phosphate, tartrate, or oxalate. The final pH of the suspensions, previously adjusted to 5.5, was recorded. The suspensions were centrifuged and filtered, and phosphate, tartrate, and oxalate concentrations in the solutions were then determined. Competitive adsorption between tartrate and oxalate One hundred milligrams of the complexes were shaken for 24 h at 20~ with a series of 25 ml of 0.02 mol 1-1 KC1 solutions containing different amounts (01.6 mmol 1-1) of tartrate and oxalate. The total amount of tartrate and oxalate added to the clay was kept constant at 400 mmol kg -1 with the oxalate/tartrate molar ratio (Ri) ranging from 0.2 to 5.0. Some samples were prepared by adding 50, 100, 150 mmol kg 1 of tartrate and oxalate while keeping Ri = 1. The suspensions were centrifuged and filtered, and tartrate and oxalate concentrations in the solutions were then determined. Competitive adsorption between phosphate and tartrate One hundred milligrams of the complexes were shaken for 24 h at 20~ with a series of 25 ml of 0.02 mol 1-1 KC1 solutions containing different amounts of phosphate and tartrate. The amounts of phosphate initially added to the clay were 150 or 400 mmol kg -1 for A1Mtl.6 and 150 or 500 mmol kg -l for A1Mt6, which resulted in nearly maximum phosphate adsorption. The molar ratio of tartrate/phosphate ranged from 0.25 to 3.0. To investigate the effect of sequence of anion addition on phosphate adsorption, tartrate was introduced either simultaneously with phosphate (P + T systems), or 2 h before phosphate (T/P systems). The suspensions were centrifuged and filtered, and phosphate and tartrate concentrations in the solutions were then determined. Competitive adsorption among phosphate, tartrate, and oxalate One hundred milligrams of the complexes were shaken for 24 h at 20~ with a series of 25 mi of 0.02 mol 1-1 KC1 solutions containing different amounts of phosphate, tartrate, and oxalate. The amounts of phosphate initially added to the clays ranged from 100 to 500 mmol kg 1, whereas those of tartrate and oxalate ranged from 50 to 375 mmol kg 1. In most samples, the total amount of phosPhate, tartrate and oxalate was kept constant at 600 mmol kg -1. The oxalate/tartrate molar ratio was 1 in all the samples, but the (tartrate + oxalate)/phosphate molar ratio ranged from 0.2 to 5. Tartrate and oxalate were introduced either simultaneously with phosphate (P + T + OX systems) or 2 h before phosphate (T + OX/P systems). The suspensions were centrifuged and filtered, and phosphate, tartrate, and oxalate concentrations in the solutions were then determined.