PHOSPHATE ADSORPTION AND THE DISPLACEMENT OF STRUCTURAL SILICON IN AN ALLOPHANE CLAY

Phosphate was adsorbed at a constant pH, on allophane clay samples which had been pre-treated with selenite to remove adsorbed silicate. The concomitant release of selenium, silicon, and hydroxyl ion on phosphate adsorption was also measured. The results are compared with those obtained from a similar experiment conducted using clay which had not been treated and they substantiate the earlier conclusion that phosphate displaces structural silicon of clays. The amount displaced increases parabolically with increasing levels of phosphate adsorbed over the concentration range studied.     

The effects of sowing depth and flooding on the emergence, survival, and growth of Fimbristylis miliacea (L.) Vahl

Glasshouse experiments were conducted from March to July 2003 to determine the effects of sowing depth, and the time, duration, and depth of flooding on the emergence, survival, and growth of Fimbristylis miliacea (L.) Vahl. The treatments that were evaluated in the first experiment were three seeding depths (0, 0.5, and 1.0 cm), while in the second experiment, three flooding depths (saturated soil with no standing water and soil with water depths of 5 and 10 cm) and three flooding durations (7, 14, and 21 days from sowing) were evaluated. In the third experiment, three flooding depths (saturated soil with no standing water, and soil with water depths of 5 and 10 cm) and four flooding regimes (7, 14, 21 or 28 days after sowing [DAS]) were evaluated. Surface seeding gave the highest emergence rate compared to the 0.5 cm and 1.0 cm soil depths. A significantly higher emergence rate was recorded with the saturated conditions than with the flooded conditions. A flooding duration of ≥ 14 days showed a clear trend of reduced emergence with increasing flooding depth. A significantly higher survival rate, plant height, root length, number of leaves, and dry matter were recorded at soil saturation followed by the 5 cm and 10 cm flooding depths when flooding was simulated at 7 and 14 DAS. When the flooding was delayed to 21 and 28 DAS, the 10 cm flooding depth was required to suppress this weed. The results provide sufficient evidence to confirm that from deeper seed burial (1 cm sowing depth), flooding depths of ≥ 5 cm of durations of 14 and 21 days and at the onset of flooding within 14 DAS were effective in suppressing the emergence and growth of F. miliacea .     

IDENTIFICATION OF PHOSPHATE-REACTIVE SITES OF HYDROUS ALUMINA FROM PROTON CONSUMPTION DURING PHOSPHATE ADSORPTION AT CONSTANT pH VALUES

The amounts of hydroxyl ions released from hydrous alumina during phosphate adsorption were measured at constant pH values and at different phosphate concentrations using an automatic titrator. The hydroxyl ions released were plotted against phosphate adsorbed and the equations that gave best fit to the data were differentiated. The variation in the differentials (dOH/dP) with increasing phosphate adsorption was consistent with the theory, proposed below, on the type of sites responsible for phosphate adsorption, by synthetic hydrous alumina containing bayerite and pseudoboehmite, at different concentrations. At low phosphate adsorption which corresponded to low phosphorus concentration, adsorption took place mainly by displacing aquo groups (Al-H₂O). With increase in phosphorus concentration hydroxo groups (Al-OH) became the predominant sites of adsorption. At still higher concentrations, where phosphate adsorption displacing aquo and hydroxo groups was nearly completed, the hydroxyl bridges linking aluminium atoms (Al-OH-Al, ol groups) were broken. This created new sites which were responsible for additional adsorption of phosphate at these concentrations.     

PHOSPHATE ADSORPTION BY SYNTHETIC AMORPHOUS ALUMINOSILICATES

Phosphate adsorption isotherms were determined for four synthetic amorphous aluminosilicate gels with A1: A1 + Si molar ratios of 0.29 to 0.88. The concomitant silicate release and acid consumed to maintain the pH of the suspensions constant were also measured. The adsorption isotherms were analysed applying a two-term Langmuir equation–assuming two types of sites. The experimental points fitted the predicted adsorption curves only up to a certain amount of phosphate adsorbed. The deviation at high phosphate adsorption values suggested the presence of more than two types of adsorption site. A comparison of phosphate adsorbed with the silicate released and acid consumed to maintain the pH constant indicated that, for a 3 h reaction time at concentrations below about 10 μmol cm^?3, phosphate exchanges mainly with aquo and hydroxo ligands and with adsorbed silicate. At higher concentrations phosphate is adsorbed (i) on sites arising from the disruption of hydroxy aluminium polymers in the gels and (ii) by the displacement of structural silicate.     

ADSORPTION OF SELENITE, PHOSPHATE AND SULPHATE ON HYDROUS ALUMINA

Selenite was adsorbed on a positively charged hydrous alumina at a solution pH of 5.0, and the OH ^? released and changes in the surface charge were measured. The adsorption isotherm levelled off at high concentrations suggesting a definite adsorption maximum. The OH^? released yielded a curvilinear relationship of increasing slope with the selenite adsorbed. The positive charge on the alumina surface was neutralized by the adsorbed selenite and the net charge became close to zero as the adsorption of selenite was near maximum. The molar ratio of the ligands (OH^?+ OH₂) displaced over selenite adsorbed was more than one at surface saturations of <0.5, but it was approaching one with increasing adsorption of selenite. The results are explained in terms of preferential adsorption of divalent (SeO²₃^?) and monovalent (HSeO^?₃ selenite on a highly positively and less positively charged surface respectively, in agreement with the VSC^? VSP model of Bowden et al. (1973; 1977). The supply of SeO²₃^? for adsorption is considered to be favoured by an expected lower H activity in solution adjacent to the positively charged surface than that in the bulk solution. A general discussion is given on the ligand exchange adsorption of selenite, phosphate and sulphate on hydrous alumina.     

Growth behavior of itchgrass (Rottboellia cochinchinensis) in Peninsular Malaysia

A glasshouse experiment was conducted at University Putra Malaysia, Serdang, to study the growth pattern of itchgrass collected from 13 locations. The itchgrass was either associated with crops or found along roadsides in six infested estates located in Johor, Kedah, Pahang, Perak, Perlis and Selangor. Significant differences between populations were observed in all variables except in time to seedling emergence. Cluster analysis of the data classified the populations into three major groups. The first group comprised populations from Johor, Pahang and Perak found in association with banana, cocoa and oil palm plantations, or along the roadsides. The second group comprised populations from Kedah, Perlis and Selangor found in association with maize, upland rice, rubber, sugar cane and tobacco plantations. The third group was a distinct population found only along roadsides at Padang Kartong, Perlis. Principal component analysis was performed to determine the growth characteristics that separated the groups. The results showed that plant height, number of leaves, number of seeds/plant, secondary culm diameter and the weight of 100 intact or dehulled seeds were the most significant characteristics that differentiated the groups. The study provided strong evidence of the existence of three itchgrass groups in Peninsular Malaysia.