Behaviour and dynamics of di-ammonium phosphate in bauxite processing residue sand in Western Australia--II. Phosphorus fractions and availability.
OBJECTIVE
The production of alumina involves its extraction from bauxite ore using sodium hydroxide under high temperature and pressure. This process yields a large amount of residue wastes, which are difficult to revegetate due to their inherent hostile properties--high alkalinity and sodicity, poor water retention and low nutrient availability. Although phosphorus (P) is a key element limiting successful ecosystem restoration, little information is available on the availability and dynamics of P in rehabilitated bauxite-processing residue sand (BRS). The major aim of this experiment was to quantify P availability and behaviour as affected by pH, source of BRS and di-ammonium phosphate (DAP) application rate.


METHODS
This incubation experiment was undertaken using three sources of BRS, three DAP application rates (low, without addition of DAP; medium, 15.07 mg P and 13.63 mg N of DAP per jar, 100 g BRS; and high, 30.15 mg P and 27.26 mg N per jar, 100 g BRS), and four BRS pH treatments (4, 7, 9 and 11 (original)). The moisture content was adjusted to 55% water holding capacity and each BRS sample was incubated at 25 degrees C for a period of 119 days. After this period, Colwell P and 0.1 M H(2)SO(4) extractable P in BRS were determined. In addition, P sequential fractionation was carried out and the concentration of P in each pool was measured.


CONCLUSIONS
A significant proportion (37% recovered in Colwell P and 48% in 0.1 M H(2)SO(4) extraction) of P added as DAP in BRS are available for plant use. The pH did not significantly affect 0.1 M H(2)SO(4) extractable P, while concentrations of Colwell P in the higher initial pH treatments (pH 7, 9 and 11) were greater than in the pH 4 treatments. The labile fractions (sum of NH(4)Cl (AP), bicarbonate and first sodium hydroxide extractable P (N(I)P)) consisted of 58-64% and 70-72% of total P in the medium and high DAP rate treatments, respectively. This indicates that most P added as DAP remained labile or moderately labile in BRS, either in solution, or in adsorbed forms on the surface of more crystalline P compounds, sesquioxides and carbonate, or associated with amorphous and some crystalline Al and Fe hydrous oxides. In addition, differences in the hydrochloric acid extractable P and the residual-P fractions among the treatments with and without DAP addition were relative small comparing with other P pools (e.g., NaOH extractable P pools), further indicating the limited capacity of BRS for fixing P added in Ca-P and other most recalcitrant forms.


CONCLUSIONS
P availability in the original BRS without addition of DAP was very low, mostly in recalcitrant form. It has been clearly demonstrated that significant proportions of P added as DAP could remain labile or moderately labile for plant use during the rehabilitation of bauxite-processing residue disposal areas. There was limited capacity of BRS for fixing P in more recalcitrant forms (e.g., Ca-P and residual-P). Concentrations of most P pools in BRS increased with the DAP application rate. The impact of the pH treatment on P availability varied with the type of P pools and the DAP rate.


CONCLUSIONS
It is recommended that the development of appropriate techniques for more accurate estimation of P availability in BRS and the quantification of the potential leaching loss of P in BRS are needed for the accurate understanding of P availability and dynamics in BRS. In addition, application of organic matters (e.g., biosolids and biochar, etc.) to BRS may be considered for improving P availability and buffering capacity.
Chen
C R
CR
Environmental Futures Centre and Griffith School of Environment, Griffith University, Nathan, QLD, 4111, Australia. c.chen@griffith.edu.au
Phillips
I R
IR
Wei
L L
LL
Xu
Z H
ZH
D000537:Aluminum Oxide / Q000737:chemistry*; D000643:Ammonium Chloride / Q000032:analysis / Q000737:chemistry; D001639:Bicarbonates / Q000032:analysis / Q000737:chemistry; D002617:Chemical Industry; D004784:Environmental Monitoring; D005290:Ferric Compounds / Q000032:analysis / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007220:Industrial Waste; D009566:Nitrates / Q000032:analysis / Q000737:chemistry; D009584:Nitrogen / Q000032:analysis / Q000737:chemistry; D010710:Phosphates / Q000032:analysis / Q000737:chemistry*; D010758:Phosphorus / Q000032:analysis* / Q000737:chemistry*; D011189:Potassium Chloride / Q000737:chemistry; D000644:Quaternary Ammonium Compounds / Q000032:analysis / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry*; D013696:Temperature; D013997:Time Factors; D014835:Volatilization; D014904:Western Australia
D001639:Bicarbonates; D005290:Ferric Compounds; D007220:Industrial Waste; D009566:Nitrates; D010710:Phosphates; D000644:Quaternary Ammonium Compounds; C024788:ammonium phosphate; D000643:Ammonium Chloride; C000499:ferric oxide; D010758:Phosphorus; D011189:Potassium Chloride; D012822:Silicon Dioxide; D000537:Aluminum Oxide; D009584:Nitrogen