Reaction mechanisms of phosphate with Al(OH)3 and a sandy soil

Abstract

Al(OH) 3 is a very effective sorbent for orthophosphate especially at low pH. At low phosphate concentration c p , phosphate is adsorbed by an exchange mechanism with singly coordinated OH(H) groups residing on the surface of the Al(OH) 3 . In chapters 2 and 3 experiments are described using a well defined gibbsite (crystalline Al(OH) 3 ) as sorbent. A rate equation isselected that describes the adsorption reaction for a fractional coverage (amount adsorbed/maximum adsorption capacity) between 0.5 and 1.0. At intermediate and high c p values the sorption can easily exceed the maximum exchange-adsorptioncapacity. The maximum adsorption capacity of the gibbsite used was estimated with the help of shadowed electron micrographs. The sorption beyond the exchange adsorption maximum is identified as a precipitation process, i.e. it only occurs in supersaturated system . The rate of this precipitation reaction is shown to increase with increasing supersaturation and to decrease with in creasing amounts sorbed. Evidence is presented that indicates that the decrease of the reaction rate with increasing sorption is caused by the formation of coatings of a (K)-Al-phosphate on the gibbsite particles. The activation energy of this reaction is determined. The ratio of the OH that is produced during this sorption reaction and the phosphate sorption isfound to be independent of the extent of the reaction, for certain reaction conditions. A so called phosphato-stat (c p -stat) has been constructed that provides for both a constant pH and c p during the sorption reaction. The amount precipitated is found proportional with the square root of reaction time for the condition of constant supersaturation.The relative contribution of precitation and adsorption has been calculated as a function of c p and reaction time. It is furthermore shown that the nature and concentration of the accompanying cation has an effect on the precipitation rate (constant total electrolyte concentration).In chapter 4 experiments are described using X-ray amorphous Al(OH) 3 and αAl 2 O 3 as sorbent. A pure phosphate solution and a so called inorganic synthetic sewage water medium are used as electrolyte solution. There is sow effect of the electrolyte medium on the sorption rate as expected. The gene ral characteristics of the sorption reaction described in this chapter are similar to the results as found with gibbsite; in the initial stage of the reaction adsorption is the dominant reaction mechanism, whereas precipitation dominates the kinetics for longer reaction times.In chapter 5 experiments are described that make use of columns either filled with quartz sand or quartz sand mixed with X-ray amorphous Al(OH) 3 . Raw domestic sewage water (pH ≈8) was added to the columns once every two weeks during 10 months. It is shown that the sorption of phosphate from sewage water in the Al-containing column is higher than would be expected from pure inorganic system experiments. Evidence is presented that indicates that a CaAl-phospate has been formed in the Al-containing sand column (Ca was absent in the pure system experiments to prevent the complication of the formation of calciumphosphates). The quartz sand columns acted as a sieve for solid compnents of the sewage water only.Experiments using an acid sandy soil with a low organic matter content as sorbent for phosphate are described in chapter 6. The main characteristics of the sorption reaction as determined from the experiments using Al(OH) 3 were also found in the sorption experiments using this soil. Use has been made of the phosphato-stat to provide conditions of constant supersaturation during the reaction. It is shown beyond doubt that the reaction rate continuously decreases at constant supersaturation as the reaction proceeds. The components of the soil that are active in the phosphate sorption process are most probably the (amorphous) aluminum-iron-oxides-hydroxides of the soil. The amount of extractable iron and aluminum was determined.The mechanism of the phosphate sorption reaction using metal oxides as sorbent is compared with the oxidation mechanism of metals.The phosphato-stat method may be used in combination with a (micro) computer, facilitating the collection of phosphate sorption rate data on a somewhat larger scale for practical applications.<p/