Research was conducted on the binding of phosphorus by crystalline iron oxides in North sea sediments, a part of the north-west European shelf, which has water a water depth ranging from under 30 metres to 200 m. the iron oxides, hydroxides and oxyhydroxides (Fe oxides) are capable of providing sorption sites for compounds with a high affinity for surfaces such as silica and phosphorus, with high affinity for the Fe oxide. The oxidized surface layer of the sediment has Fe oxides that trap pore water HPO2-4 diffusing upwards (Krom and Berner 805).
There is fast sorption and release of Phosphorus from the Fe oxides which helps to regulate the concentration of pore water HPO2-4. This has a direct effect on the sediment-water exchange, and the release of phosphorus and fluoride from Fe oxides at the same time can provide an atmosphere for the early precipitation of digenetic carbonate fluoropatite (Ruttenberg and Berner 993).
Formation of iron oxide in the marine environment
One of the main sources of iron oxide in the sea is Abandon Mine Drainage (AMD) from coal mining regions. Such occurrences have been observed in Pennsylvania’s four main river basins, whose abandoned coal mines have led to the contamination of numerous numbers of streams, whereby the AMD pollutants degrade the water quality as their substrates result in adverse effects on the aquatic ecology and designated uses. The AMD is formed from a series of complex geo-chemical and microbial reactions that occur due to contact between oxygenated water and pyrite (FeS2), present in the coal, refuse or overburden, involved in mining operations (Kang, Kwang-Ho and Kwang-Hee 3859).
The pH of AMD varies in the acidic region, between 2.5 and 6. It contains dissolved metals like iron, manganese and aluminium. The concentration as well as composition of the metal ions is dependent on the pH of the AMD. The iron oxide precipitate is formed from various chemical reactions involving pyrite weathering, oxidation of ferrous iron and hydrolysis. The general reaction can be expressed in the equation below.
4 FeS2 (s) + 15 O2 (g) + 14 H2O → 4 Fe (OH)3 (s) + 8 H2SO4 (aq)
When the iron (Fe) is in water, it undergoes two reactions namely: oxidation, a process that leads to a decrease in acidity; and hydrolysis, a process that increases acidity. The equations for the reactions are as below (Kang, Kwang-Ho and Kwang-Hee 3859).
Fe2+ + ¼ O2 + H+ → Fe3+ + ½ H2O (oxidation)
Fe3+ + 3H2O → Fe (OH)3 + 3H+ (hydrolysis)
Research conducted on the water has indicated that alkalinity and acidity can both be present in the water. Tests can be undertaken to determine the actual pH, which is either net acidity or net alkalinity. In a ‘hot’ acidity measurement, a value less than 0 mg/l indicates more acidity, while that more than 0 mg/l indicates net alkalinity. Another procedure would include deducting the stoichiometric acidity of the metals from the alkalinity. A value below 0 mg/l indicates net acidity, while that above 0 mg/l indicates net alkalinity in the water (Kang, Kwang-Ho and Kwang-Hee 3860).
The latter has adequate alkalinity to neutralize the mineral acidity that is a product of oxidation and hydrolysis processes of the metal ions. There is reduced concentration of iron resulting in reduced iron oxide precipitates, when the oxidation and hydrolysis reactions occur in aerobic environments. AMD treatment options use the oxidation and hydrolysis reactions of iron, to obtain by-products such as iron oxide solids, ferrihydtite, which contains few contaminants (Kang, Kwang-Ho and Kwang-Hee 3862).
Reaction of iron oxide with phosphorus
Iron oxides have a high affinity for phosphates. In experiments conducted to determine the adsorption of phosphate to synthetic iron oxides in effluent water, a model using Freundlich isotherm was deduced, as below.
Q = K Cn 6
Where Q is the amount of phosphorus adsorbed at equilibrium per gram of iron oxide, C is the equilibrium solution phase concentration of phosphorus, and K and n are empirically determined parameters. From the equation, it was determined that AMD iron oxides were beneficial in applications involving the treatment of wastewater, due to its high capacity for phosphate adsorption.
Iron oxide is one of the common metallic oxides used in the treatment of waste water, to remove phosphorus from effluent stream. To obtain the iron oxide, Ferric chloride, FeCl3 is hydrolyzed, in a chemical reaction as below (Lambertus 538).
FeCl3 + 6 H2O → Fe (OH)3 + 3 H3O+
The amorphous ferrihydtite is observed to contain a large surface area that seizes phosphorus via reactions that include precipitation and sorption on the surface of the particle via the reactions below (Lambertus 539).
Fe-OH + H2PO4– → Fe-O-H2PO3 + –OH
This is mainly influenced by the surface area and surface charge of the particles. Ferihydrate that has been freshly precipitated has a surface area of about 250 m2/g. This gives it a large capacity to bind phosphates. Goethite and hematite are semi-crystalline iron oxides whose binding capabilities are much lower, due to their small surface areas, nearly ten times lower than ferrihydtite. Research conducted on the three iron oxides indicated that they had a similar binding constant, when the surface area was adjusted. The research indicated that the binding sites for iron oxides did not vary. In addition to this, it was discovered that phosphate binding occurred via a general mechanism that was not affected by the crystallinity or crystal structure (Parfitt, Atkinson and Roger 838).
Factors affecting the bonding process
The sorption of ions to the surface of iron oxide is dependent on the surface charge. The surface charge, on the other hand is dependent on pH and ionic strength of the solution. With regard to this, anions like phosphate indicate a strong pH dependent binding to the surface of iron oxides. Experiments conducted to identify the sorption rate of phosphate to ferrihydtite in buffered deionised water showed a decrease by up to 50%, when the pH was adjusted by a single unit (Parfitt, Atkinson and Roger 840).
Fe-oxide levels in the marine environment
Many marine sediments have increasing amounts of Fe oxides with depth, which enables Fe-bound phosphorus to be a vital reservoir for phosphorus in marine sediments. HPO2-4 sorption and susceptibility to reduction aspects are influenced by the mineralogy and crystallinity of Fe oxides. Knowledge of Fe oxides in the marine environment mainly refers to concretions of nodules found in deep sea sediments. The mineral forms of Fe oxides in coastal marine sediments can be determined using conventional techniques such as X-ray microanalysis and X-ray powder diffraction, though the procedures are difficult due to a few factors including their low concentrations, poor crystallinity, and their existence as coatings on other particles (Stomp and Van Raaphorst 478).
Alternative extraction techniques involve selectivity using pure mineral phases. The techniques are operationally defined when applied to natural materials. This is because extractants lack absolute specificity in mineral or phase separation. In addition to this, mineral phases in sediments may have varying solubility compared to standard materials used for calibration, usually because of a different mod and environment of formation, as well as variations in the magnitude of weathering prior to deposition (Stomp and Van Raaphorst 479).
Kang, Suck-Ki, Choo Kwang-Ho and Lim Kwang-Hee. “Use of Iron Oxide Particles as Adsorbents to Enhance Phosphorus Removal from Secondary Wastewater Effluent.” Separation Science and Technology (2003): 38, 3853-3874.
Krom, M. D. and R. A. Berner. “Adsorption of phosphate in anoxic marine sediments.” Limnol. Oceanogr. (1980): 25, 797-806. Print.
Lambertus, Lijklema. “Interaction of orthophosphate with iron (III) and aluminum hydroxides.” Environmental Science and Technology (1980): 14, 537-541.
Parfitt, Roger L., Roger J. Atkinson and C. Smart Roger. “The mechanism of phosphate fixation by iron oxides.” Soil Sci. Soc. Am. Proc. (1975): 39, 837-841.
Ruttenberg, K. C. and R. A. Berner. “Authigenic apatite formation and burial in sediments from non-upwelling, continental margin environments.” Goechim. Cosmochin. Acta (1993): 57, 991-1007.
Stomp, C. P. and W. Van Raaphorst. “Phosphate adsorption in oxidized marine sediments.” Chem. Geol. (1993): 107, 477-480. Print.