Phosphate Concentration In Solution Research Articles

Characteristics and mechanisms of phosphate adsorption on dewatered alum sludge

The adsorption characteristics of phosphate adsorption on the dewatered alum sludge were identified as a function of pH and ion strengths in solution. In addition, adsorption mechanisms were investigated by conducting batch tests on both the hydrolysis and P-adsorption process of the alum sludge, and making a comparative analysis to gain newer insights into understanding the adsorption process. Results show that the adsorption capacity decreased from 3.5 to 0.7 mg P/g sludge when the solution pH was increased from 4.3 to 9.0, indicating that adsorption capacity is largely dependent upon the pH of the system. The results of the competitive adsorption between phosphate and typical anions found in wastewater, such as SO 4 2− and Cl −, onto alum sludge reveal that alum sludge can selectively adsorb phosphate ions. The insignificant effect of SO 4 2− and Cl − on P-adsorption capacity indicates that phosphate adsorption is through a kind of inner-sphere complex reaction. During the adsorption process, the decrease of phosphate concentration in solution accompanied with an increase in pH values and concentrations of SO 4 2−, Cl − and TOC (total organic carbon) suggests that phosphate replaced the functional groups from the surface of alum sludge which infers that ligand exchange is the dominating mechanism for phosphate removal. At the same time, the simultaneous decreases in PO 4 3− and total aluminium concentration in solution indicate that chemical reaction and precipitation are other mechanisms of phosphate removal.

Binding of inorganic sulphate and phosphate in humid-climate soils measured using column leaching, equilibration and extraction methods

The ability of soil samples to bind applications of inorganic sulphate and phosphate was examined in the laboratory using column leaching, solution equilibration and extraction methods. Eleven soil samples from the Fraser Valley of British Columbia with a range of soil characteristics and management histories were examined. Column leaching trials showed that sulphate was bound to soils, but not as strongly as phosphate. There was no evidence of significant sulphate binding during the equilibration trials but there was substantial native sulphate bound by the soils as shown by extraction of significant sulphate concentrations by bicarbonate extraction following equilibration. This shows that sulphate is not bound according to the traditional theory of adsorption, but other undefined mechanisms are involved. Both column leaching and equilibration results showed that the samples studied varied considerably in their native phosphate retention attributes, with some samples releasing no phosphate and others releasing considerable phosphate into the water treatments. Equilibration trials showed that some of the soils bound large quantities of phosphate resulting in no phosphate in the soil solution whereas others bound much less resulting in significant phosphate concentrations in the soil solution. Phosphate binding followed the Freundlich equation in eight of the samples, the Tempkin equation in one sample and a direct linear relationship in two samples. The Langmuir equation fitted most of the samples that had followed Freundlich equation reasonably well, but did not fit the other samples. This showed that determination of the maximum amount of phosphate that can be bound by the sample (i.e., P saturation) using the Langmuir equation cannot be used for all humid-climate soils of British Columbia. Two alternate direct measurements are proposed to determine the potential risk of phosphate in soils to contaminate water: amount of phosphate released during equilibration with water (native unbound phosphate) and amount of phosphate bound during equilibration with a low (approximately 25 mg P kg-1) phosphate solution (potential for the soil to bind applied P). These results show that sulphate and phosphate binding mechanisms by coastal British Columbia soils are not well understood. The nature of the binding mechanisms needs to be determined to develop and interpret soil analyses for agronomic and environmental purposes. Key words: Sulphate binding, phosphate binding, P saturation, soil solution P

Isotherms and kinetic study of dihydrogen and hydrogen phosphate ions (H2PO4-and HPO42-) adsorption onto crushed plant matter of the semi-arid zones of Morocco:Asphodelus microcarpus,Asparagus albusandSenecio anthophorbium

In the present work H 2 PO - 4 and HPO 2- 4 ions adsorption onto organic matter (OM) obtained from ground dried three plants growing in arid zones of Morocco has been studied. The adsorption process is affected by various parameters such as contact time, particle size and initial concentration of phosphate solution (C i ≤30 mg/l). The uptake of both ions is increased by increasing the concentration of them selves. The retention of phosphate ions by Asphodelus microcarpus, Asparagus albus are well defined by several isotherms such as the Langmuir, Temkin and Freundlich.

Nutrient uptake by rice and soil solution composition under atmospheric CO2enrichment

Using free-air CO2 enrichment (FACE) we grew rice crops at ambient or elevated (ca. 250 μmol mol−1 above ambient) and evaluated soil nutrition status by determining the elemental composition of soil solution. The dry matter of rice was increased by elevated CO2. Although the increase in dry matter after panicle initiation was greater in rice grown with FACE than in rice grown in ambient CO2, the increase rate was lower after panicle initiation. The nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) concentrations of rice were significantly 21, 6, 14, and 9% lower, respectively, in rice grown with FACE than the concentrations in ambient grown rice at panicle initiation, and the N and K concentrations were significantly lower with FACE at harvest. Although the N and K uptakes of rice were higher with FACE than in ambient grown rice at panicle initiation, the differences were small after panicle initiation. The ammonium, phosphate, K, and silicate concentrations in soil solution gradually decreased before panicle initiation. The relation between the N and K uptake of rice and the concentrations in soil solution indicated the N and K supply from soil regulated the uptake after the concentrations were decreased in soil solution. The N uptake of rice appeared to be determined by the soil N availability, not by nutrition demand and uptake ability of rice after panicle initiation, and the amount of N uptake seemed to limit rice growth.

An investigation of phosphate adsorbed on aluminium oxyhydroxide and oxide phases by nuclear magnetic resonance

SummaryThe adsorption of phosphate by soil minerals controls availability of P to plants, but the chemical environments of adsorbed phosphate are poorly known. We used 31P MAS NMR to study the adsorption of phosphate on to boehmite (γ‐AlOOH) and γ‐Al2O3 with large surface areas. The solid phases were reacted in 0.1 m phosphate solutions at pH from 3 to 11 and in solutions with pH 5 at concentrations from 10−1 m to 10−4 m. The spectra suggested three different phosphate environments: (i) orthophosphate precipitated from the residual solution after vacuum filtering, (ii) surface‐adsorbed phosphate in inner‐sphere complexes, and (iii) Al‐phosphate precipitates on the surfaces of the minerals. The chemical shifts of both the inner‐sphere complexes and surface precipitates became progressively less shielded with increasing pH and decreasing concentration of phosphate solution. For the inner‐sphere complexes, we interpret these changes to be the result of decreasing phosphate protonation combined with rapid proton exchange among phosphate tetrahedra with different numbers of protons, which causes peak averaging. The chemical shifts of 31P of the Al‐phosphate precipitates were more negative than those of the surface phosphates at a given pH and solution concentration, probably because of a larger number of P–O–Al linkages per tetrahedron. The observed trend of decreasing shielding is probably due to the decreasing average number of P–O–Al linkages per tetrahedron combined with decreasing protonation and an increasing number of K+ next‐nearest neighbours. Even at small concentrations of phosphate solution, a significant amount of Al‐phosphate precipitate was present.

Mechanism of flavin mononucleotide cofactor binding to the Desulfovibrio vulgaris flavodoxin. 1. Kinetic evidence for cooperative effects associated with the binding of inorganic phosphate and the 5'-phosphate moiety of the cofactor.

The pathway(s) by which the flavin cofactor binds to the apoflavoprotein is the subject of some debate. The crystal and NMR structures of several different flavodoxins have provided some insight, although there is disagreement about the location of the initial interaction between the flavin mononucleotide (FMN) and the apoflavodoxin and the degree of protein conformational change associated with cofactor binding [Genzor, C. G., Perales-Alcon, A., Sancho, J., and Romero, A. (1996) Nat. Struct. Biol. 3, 329-332; Steensma, E., and van Mierlo, C. P. M. (1998) J. Mol. Biol. 282, 653-666]. Binding kinetics using stopped-flow spectrofluorimetry and phosphate competition studies were used to develop a model for flavin binding to the flavodoxin from Desulfovibrio vulgaris. In the presence of phosphate, the time course of fluorescence quenching associated with FMN binding to apoflavodoxin was biphasic, whereas riboflavin, which lacks the 5'-phosphate group of FMN, displayed monophasic binding kinetics. When the concentration of phosphate in solution was increased, the FMN binding rates of the two phases behaved differently; the rate of one phase decreased, while the rate of the other increased. A similar increase in the single phase associated with riboflavin binding was also observed. This has led to the following model. The binding of the flavin isoalloxazine ring to its subsite is dependent on the presence of a phosphate group in the phosphate-binding subsite. When phosphate is in the buffer solution, FMN can bind in either of two ways: by the initial insertion of the 5'-phosphate group followed by ring binding or, when inorganic phosphate from solution is bound, the insertion of the isoalloxazine ring first. Riboflavin, which lacks the phosphate moiety of FMN, binds only in the presence of inorganic phosphate, presumably due to the binding of this group in the phosphate-binding subsite. These results suggest that cooperative interactions exist between the phosphate subsite and the ring-binding region in the D. vulgaris flavodoxin that are necessary for isoalloxazine ring binding.

Plant roots release phospholipid surfactants that modify the physical and chemical properties of soil.

• Plant root mucilages contain powerful surfactants that will alter the interaction of soil solids with water and ions, and the rates of microbial processes. • The lipid composition of maize, lupin and wheat root mucilages was analysed by thin layer chromatography and gas chromatography-mass spectrometry. A commercially available phosphatidylcholine (lecithin), chemically similar to the phospholipid surfactants identified in the mucilages, was then used to evaluate its effects on selected soil properties. • The lipids found in the mucilages were principally phosphatidylcholines, composed mainly of saturated fatty acids, in contrast to the lipids extracted from root tissues. In soil at low tension, lecithin reduced the water content at any particular tension by as much as 10 and 50% in soil and acid-washed sand, respectively. Lecithin decreased the amount of phosphate adsorption in soil and increased the phosphate concentration in solution by 10%. The surfactant also reduced net rates of ammonium consumption and nitrate production in soil. • These experiments provide the first evidence we are aware of that plant-released surfactants will significantly modify the biophysical environment of the rhizosphere.

Comparison of phosphate fertilizers for pasture and their effect on soil solution phosphate

A field experiment was conducted over a period of six years to evaluate the agronomic effectiveness on permanent pasture of (i) an unground North Carolina phosphate rock (PR), and (ii) the PR treated with elemental sulfur (S) and Thiobacillus spp. bacteria (PR/S). Single superphosphate (SSP) was used as the reference fertilizer. Changes in soil solution phosphate (Pi) with time at 0–20 mm, 20–40 mm, and 40–75 mm depths, as effected by fertilizer treatments at different rates, were also determined. Application of PR/S gave the same dry matter yield as SSP from the first year onwards whereas PR gave less dry matter yield than SSP in the first 3 years and then again in year 5. The differences between fertilizers were not significant in years 4 and 6. Phosphate concentration in soil solution in the control treatment was greater in summer than in spring as a result of low pasture growth in summer, which was due to low rainfall. On application of SSP, Pi concentration in solution increased with increasing rate of application and the values rapidly declined with time. At 30 kg P ha−1 rate, Pi in solution returned to similar concentration to that of control in about 3 to 6 months after application each year and there appeared to be no build up in soil labile phosphorus (P) (soil P in equilibration with solution P), as indicated by solution Pi, with time after 6 annual applications. At greater rates of SSP application there was evidence of accumulation of labile P as indicated by increased solution Pi concentrations. Application of PR increased solution Pi concentration up to 90 kg P ha−1 rate and the value marginally decreased at 120 kg P ha−1 rate. Maximum Pi concentration was observed every year about 6–10 months after PR application. Application of SSP at 60 kg P ha−1 or greater rate resulted in enhanced solution Pi up to 75 mm depth of soil sampling. Application of PR at equivalent rates of P did not significantly increase solution Pi below 20 mm depth. It was concluded that for permanent pastures PR/S is as effective as SSP from the first year of application whereas PR from the fourth to sixth year.

Bone origin of the serum complex of calcium, phosphate, fetuin, and matrix Gla protein: biochemical evidence for the cancellous bone-remodeling compartment.

We previously described the discovery of a fetuin-matrix Gla protein (MGP)-mineral complex in the serum of rats treated with the bone-active bisphosphonate etidronate and showed that the appearance of this complex in serum correlates with the inhibition of bone mineralization by etidronate. In this study we show that the inhibition of bone resorption by treatment with the hormone calcitonin, the cytokine osteoprotegerin, or the drug alendronate, completely inhibits the generation of the fetuin-mineral complex in response to etidronate injection. These observations can be explained best by the bone-remodeling compartment (BRC), a cancellous bone compartment in which the concentrations of calcium and phosphate are determined directly by the combined actions of the osteoclast and the osteoblast. When bone mineralization is acutely inhibited by etidronate, the BRC model predicts that the continuing action of osteoclasts will cause a sharp rise in the concentrations of calcium and phosphate in the aqueous solution of the BRC with the consequent spontaneous formation of calcium phosphate crystal nuclei in which growth then would be arrested by formation of a complex with fetuin. When the inhibition of bone resorption by calcitonin, osteoprotegerin, or alendronate is combined with the acute inhibition of bone mineralization with etidronate, the BRC model correctly predicts that there will no longer be a sharp rise in calcium and phosphate, and, therefore, there will no longer be the formation of the fetuin-mineral complex. The vascular nature of the BRC is supported by the observations that the fetuin component of the fetuin-mineral complex is derived from plasma fetuin and that the fetuin mineral complex appears in plasma within minutes of the inhibition of bone mineralization with etidronate.

Cellular biosensing system for discovery of protein synthesis inhibitors with an electrochemical phosphate modulator to regulate the acid phosphatase gene expression of Saccharomyces cerevisiae

A cellular biosensing system for screening protein synthesis inhibitors has been developed by linking an electrochemical phosphate modulator and matrix-immobilized yeast cells with an optical sensing device. To screen the protein synthesis inhibitors, yeast phosphatase gene regulating system has been employed by linking an electrochemical phosphate modulator. Since the yeast phosphatase gene coding γAPase is expressed, when the phosphate concentration in solution is lowered below the threshold, the γAPase production is triggered by lowering the phosphate concentration with the electrochemical phosphate modulator, and monitored continuously with the photometric device. The electrochemical phosphate modulator was assembled with matrix-immobilized yeast cells. The module could insert to ordinal cuvette to monitor the induced γAPase activity in an ordinal photometer. Using the system, induction profile of protein synthesis was easily observed and was affected remarkably by various protein synthesis inhibitors. This seems promising that the system can be applied for first screening process of de novo protein inhibitors. The cellular biosensing system seems promising in screening protein synthesis inhibitors.

Uptake of P from apatite by Pinus sylvestris seedlings colonised by different ectomycorrhizal fungi

The role of ectomycorrhizal (EM) fungi in increasing apatite dissolution was tested in a pot system with Pinus sylvestris (L.) seedlings growing in a sand/peat mixture. Non-mycorrhizal seedlings and seedlings inoculated with one of three different isolates of EM fungi were grown for 210 days in pots divided into a root-containing and a root-free compartment. The EM mycelium was allowed to colonise the root-free compartment, to which apatite had been added as a P source in half of the pots. All mineral nutrients except P were supplied in the form of a balanced nutrient solution. Seedlings grown with apatite as their P source grew significantly better and had higher P concentrations (1.1-1.5 mg/g) compared with seedlings growing without any P source (0.6-1.0 mg/g), indicating that they were able to use apatite-P. A weathering budget based on P uptake indicated that 6.7-18.9 mg apatite was weathered per pot which corresponds to 0.3-0.9% of the added apatite. A similar budget based on Sr uptake indicated that the apatite weathering rate was lower (0.13-0.3%). One Suillus variegatus isolate and an unidentified fungus had a significant positive influence on the dissolution of apatite, while another S. variegatus isolate had colonised roots poorly and did not influence weathering significantly. Oxalic acid was detected in root-free soil and was especially abundant in compartments colonised by S. variegatus. The concentration of oxalic acid was correlated to the concentration of phosphate in the soil solution of root-free soil, indicating that oxalic acid influences apatite dissolution.

Simulation of the effect of citrate exudation from roots on the plant availability of phosphate adsorbed on goethite

SummaryRhizosphere processes strongly influence the availability of phosphorus (P) to plants. Organic ligands that are exuded from the root surface mobilize phosphorus by dissolution of P minerals or by desorption of adsorbed phosphate. We developed a mechanistic model to study the mobilization of phosphate sorbed on goethite by the exudation of citrate and consequent uptake of phosphate by the root. The use of a model allows the effects of the organic anion and pH on P desorption to be separated. The model is also used to predict concentration profiles developing around the root for phosphate, citrate (with or without accounting for degradation) and pH, providing insight into the processes that occur in the rhizosphere. Results of model calculations show that with larger rates of citrate exudation, greater P availability is predicted. Exudation at a rate of 0.5 μmol citrate m–1 root day–1, which is in the range found for P‐deficient plants, increased P availability almost 2‐fold at fairly large phosphate loading of goethite (1.9 μmol m–2) and almost 30‐fold at small phosphate loading (1.3 μmol m–2). Competitive adsorption causes a much greater relative increase in the phosphate concentration in solution at small than at large phosphate loading, which explains this result. Simultaneous acidification of the rhizosphere results in a smaller P mobilization than at a fixed pH of 5, as a result of the pH dependence of phosphate adsorption in the presence of citrate. Sorption of citrate increases its persistence against microbial decay, and hence has a positive effect on the mobilization of adsorbed phosphate.

Empirical modelling of the kinetics of phosphate sorption to macropore materials in aggregated subsoils

SummaryPreferential transport of phosphate through macropores increases the significance of phosphate sorption to macropore wall materials compared with bulk soil materials. Therefore we studied the kinetics of phosphate sorption to soil bulk materials from the Ap and Btg horizons, from the iron oxide‐poor (Albic) centres and from iron oxide‐rich (Red) exteriors of the fractures in a clayey pseudogley in batch at initial phosphate concentrations (P0) up to a maximum of 650 μm and at sampling times up to 7 days. Uptake of phosphate was least in the phosphate‐rich Ap, and the Albic material also adsorbed little. By contrast the Red material sorbed phosphate strongly. By plotting the logarithm of the solution phosphate concentration against log time, two or three different kinetic regions could be distinguished: a fast reaction within the first minute of reaction; a lag period at intermediate to large initial values of phosphate concentration (P0); and a steady slow reaction. The lag phase cannot be described satisfactorily by the well‐known adsorption models. The Langmuir equilibrium expression produced rather good fits at any fixed time of reaction, but the estimated adsorption maximum increased steadily with time. Empirical models were fitted to the data, among them a ‘lag‐linear’ model developed to include the lag phase. The relative contribution of the fast reaction to total adsorption during 7 days decreased from 50 to 80% at small P0 to less than 10% at large P0 values. The fraction of fast‐adsorbed phosphate followed the order Red > Btg > Albic > Ap regardless of P0; the slow reaction is, by a log–log rate, ranked Albic ≅ Btg > Red > Ap for P0 < 20 μm and Red > Btg > Albic > Ap at larger initial concentrations. The Albic material does not minimize the risk of phosphate leaching through macropores, as demonstrated by its small P sorption capacity, the relatively slow adsorption rate and its small actual P content.

Phosphate sorption to matrix and fracture wall materials in a Glossaqualf

Phosphate retaining properties of macropore wall materials may influence the extent of phosphate leaching via preferential flow. Phosphate sorption to soil matrix material has been compared with sorption to fracture wall materials consisting of inner iron-oxide depleted albic coatings rimmed by reddish iron-oxide enriched quasicoatings. The latter contains 15 times as much Fe oxide, 2 times as much Al (hydr)oxide and 5–6 times as much total P than the albic material, which represent the most P-depleted material in the profile. Oxalate extractable P (P o) amounts to 1 4 − 1 2 of the total P contents of the samples. Smectite predominates in the fracture walls and acid subsoils, but has partly transformed into hydroxy-interlayered smectite (HIS) in the upper limed soil horizons. Langmuir parameters derived from phosphate sorption isotherms show no correlation for sorption to whole soil samples and the corresponding clay fractions. The phosphate sorption capacity at a threshold equilibrium concentration of 10 μM (P ads t (10 μM)) is measured as the content of P o plus the amount of phosphate sorbed during 7 days, and can be related to Al o+Fe o: P ads t (10 μM)=(0.0404±0.0046)·(Al o+Fe o)***+3.569. The P ads t (10 μM) values vary between 4 and 14 mmol P kg −1, lowest in the albic fracture walls and highest in the iron enriched quasi-coatings. The sorption capacity of the metal oxide free clay does not appear to correlate with smectite or HIS contents. In total, the clay fraction contributes with about 50% of the whole soil sorption capacity in agreement with 40–60% of the total soil Al o+Fe o being present in the clay fraction. At a solution phosphate concentration of 10 μM, the Ap horizon is found to be almost phosphate saturated whereas subsoil horizons including the fractures have phosphate saturations ≤50% and, hence, can strongly sorb P. The low phosphate saturation of fracture walls indicates that they do not act as major sinks of phosphate-rich solutions from topsoil horizons, either due to kinetic constraints, or due to a flow pattern not allowing P-rich solution to percolate fractures.

Interaction of Synthetic Sulphate “Green Rust” with Phosphate and the Crystallization of Vivianite

AbstractBecause layered Fe(II)Fe(III)-hydroxides (Green rusts, GRs) are anion exchangers, they represent potential orthophosphate sorbents in anoxic soils and sediments. To evaluate this possibility, two types of experiments with synthetic sulphate-interlayered GRs (GRSO4 = Fe2+4Fe3+2(OH)12SO4 ×H2O) were studied. First, sorption of phosphate in GRSO4 was followed by reacting suspensions of pure GRSO4 synthesized by oxidation of Fe(II) with an excess of Na2HPO4 (pH 9.3). Second the possible incorporation of phosphate in GR during formation by Fe(II)-induced reductive dissolution of phosphate-containing ferrihydrites was examined in systems containing an excess of Fe(II) (pH 7). With excess phosphate in solution, GRSO4 initially sorbed phosphate in the interlayer producing a basal layer spacing of 1.04 nm, but only ~50% of the interlayer sulphate was exchanged with phosphate. This GR slowly transformed to vivianite within months. In the Fe(II)-rich systems, reaction with synthetic ferrihydrites produced GRSO4 similar to that produced by air oxidation. Reaction of Fe(II) with phosphate-containing ferrihydrites initially produced amorphous greenish phosphate containing precipitates which, after 3–4 h, crystallized to GRSO4 and vivianite. In these solutions, stable phosphate-free GRSO4 can form since precipitation of vivianite produced low phosphate activity. Consequently, in both systems GR or amorphous greenish precipitates act as reactive intermediates, but vivianite is the stable end-product limiting phosphate concentration in solution. It is also inferred that Fe(OH)2 is an unlikely phosphate sorbent in mixed Fe(II)-Fe(III) systems because GR phases are more stable (less soluble) than Fe(OH)2.

Nitrate and phosphate uptake by Agropyron desertorum and Artemisia tridentata from soil patches with balanced and unbalanced nitrate and phosphate supply

To test the ability of plants to integrate small‐scale imbalances in soil nitrate and phosphate patches, plant growth and acquisition of nitrate and phosphate were measured for the perennial grass Agropyron desertorum (Fisch. ex Link) Schult. and the shrub Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle in soil where the principal supply of nitrate and phosphate came from two enriched patches. The soil was calcareous loamy‐skeletal Typic Haploxerolls. These patches were applied in two treatments: either nitrate and phosphate were applied in both patches (balanced treatment) or one patch contained only nitrate and the other only phosphate (unbalanced treatment). The same total quantity of nutrients was applied in both treatments and these included 15N and 32P tracers. The plants were in large pots in open field conditions. There were no significant differences in total biomass production and nitrogen concentration between the two treatments, indicating that both species had the physiological ability to integrate soil nutrient resources. Artemisia was able to acquire more phosphate in the unbalanced treatment, probably due to the high local solution phosphate concentration. Generally Artemisia acquired more N and P than did Agropyron.

Speciation of Lead in Contaminated Soils

Lead contaminated soils from various origins were sampled and analyzed for total Pb content, dissolved Pb, soil pH, soil organic matter content and Mo-reactive phosphate in solution. The concentration of free Pb 2+ ionic species in solution was determined using differential pulse anodic stripping voltammetry (ASV), assuming that ASV-labile Pb excludes organo–Pb complexes. The dataset shows the solubility relationships between the various soil characteristics for 84 soils covering a wide range of soil properties; pH (3.5 to 8.1), total Pb (10 to 15 000 mg Pb kg −1), dissolved Pb (0.6 to 124 μg Pb litre −1), free Pb 2+ activity (10 −10.4 to 10 −6.8 m) and soil organic matter (4.5 to 108 g C kg -1). Non-linear regression analysis shows that free Pb 2+ in soil solution can be predicted simply using the log of total soil Pb and pH with no significant contributions from the soil organic matter content or the concentration of phosphate in solution. In the case of dissolved Pb, log of total Pb, soil pH and solution phosphate are significant parameters, but soil organic matter does not contribute to the regression. About ∼60–80% of the dissolved Pb is present as organo–Pb complexes, this percentage is lower in soils with low pH or high total Pb content.

Availability to plants of phosphate adsorbed on goethite: experiment and simulation

SummaryA model has been developed to predict the availability of sorbed phosphate to plants. It is based on uptake by a growing root system. Phosphate sorption is assumed to be reversible and the rate of desorption is assumed not to limit uptake. The model was tested against observed availability of sorbed phosphate estimated from uptake by plants growing in phosphorus‐deficient conditions. The plants were grown on quartz sand coated with goethite on which phosphate was adsorbed, and which was mixed with nutrient solution. The influence of the phosphate loading of goethite and the goethite content of the sand on available phosphate was examined. Phosphate uptake was predicted well by the model for treatments with high phosphate loading and different goethite content of the sand. However, for a treatment with low phosphate loading, uptake was underestimated by the model. In this treatment the pH of the 'soil’ solution decreased from 5·5 to 4·2 during the experiment. The phosphate concentration in solution increased with a lowering of the pH in this pH range, resulting in increased phosphate uptake.

Dissolution of powdered human enamel suspended in acid solutions at a high solid/solution ratio under a 5% C0 2 atmosphere at 20°C

The aim was to examine the nature of enamel dissolution in aqueous suspensions with a high solid/solution ratio and in a C0 2-rich atmosphere. Before experimentation, a water-saturated mixture of 95% N 2-5% C0 2 was passed through the acid solutions for 24 hr. Samples of 2 g of powdered enamel were suspended in 7 ml of either 5 or 10 mmol/l HC10 4, with or without 2 parts/10 6 fluoride and kept gently agitated for 24 hr in the above atmosphere. The same enamel samples were repeatedly exposed to fresh acid for 26 runs. All experiments were duplicated. The aqueous phase was analysed after 20 min and 24 hr for calcium, phosphate, fluoride, chloride, sodium and magnesium. It was found that after 20 min the fluoride was invariably taken up in the enamel and the solution was supersaturated with respect to hydroxyapatite with pH ranging 6.7-5.6. During the following 24 hr pH increased further, the supersaturation remained unchanged and the concentrations of calcium and phosphate in solution decreased. In contrast, sodium, magnesium and chloride were released from enamel during the entire period. In the later runs, the supersaturation with respect to hydroxyapatite was only modest and the decrease of calcium and phosphate concentrations limited, as were the release of sodium, magnesium and chloride. It is concluded that despite a C0 2-rich atmosphere, calcium, phosphate and carbonate were released from enamel and quickly established a supersaturation with respect to hydroxyapatite with a secondary reprecipitation of mineral. It indicates that within the dental caries lesion in vivo, lesion fluid cannot exist undersaturated with respect to enamel apatite.

Sensitivity of Isotopically Exchangeable Phosphate in Soil Suspensions to the Supporting Solution

AbstractThe isotopically exchangeable quantity (Et) of phosphate measured in 1:10 soil/solution suspensions with deionized water has been identified as the reserve of plant‐available phosphate in agricultural soils. We studied the effects of two supporting solutions, 0.01 M CaCl2 and deionized water, on Et values in 27 calcareous agricultural soils. For three soils, Et was also measured after adding increasing phosphate quantities as KH2PO4, and equilibrating for 24 h, to assess changes in Et with phosphate concentration in solution (Cp). In unamended and phosphate‐enriched soils, Cp(CaCl2) was smaller than and related to Cp(H2O). The initial rapidly and the subsequent slowly isotopically exchangeable phosphate were both affected by the use of CaCl2 solution. Since both the rapid and slow isotopically exchangeable phosphate are Cp dependent, the overall effect of CaCl2 solution was analyzed considering two components: (i) a nonspecific effect attributed to the decrease in Cp and (ii) a specific effect attributed to phosphate reactions (i.e., adsorption or precipitation) in calcareous soils. Modifications in Et are the result of both nonspecific and specific effects. The change in Et due to the use of 0.01 M CaCl2 instead of deionized water varies with the soil sample, and can be positive, negative or nil depending on the time of exchange. For instance, after 24 h of exchange, the ratio {[E24h(CaCl2) − E24h(H2O)]100/E24h(H2O)} ranged from −30 to 340% for the 27 soils. The 0.01 M CaCl2 solution thus introduces complex and unpredictable changes in isotopic exchanged phosphate, and its use is not recommended to estimate plant‐available phosphate in soil.