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P sorption Characteristics On Co-precipitated Amorphous Al-Fe Hydr(oxides)

P sorption Characteristics On Co-precipitated Amorphous Al-Fe Hydr(oxides). Omar Richard Harvey. Rationale. P sorption capacity strongly correlated with Al and Fe hydr(oxide) content in soils (Freese et al. 1992) and wetlands (Reddy et al. 1995).

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P sorption Characteristics On Co-precipitated Amorphous Al-Fe Hydr(oxides)

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  1. P sorption Characteristics On Co-precipitated Amorphous Al-Fe Hydr(oxides) Omar Richard Harvey

  2. Rationale • P sorption capacity strongly correlated with Al and Fe hydr(oxide) content in soils (Freese et al. 1992) and wetlands (Reddy et al. 1995). • Mixed hydr(oxides) more common than pure phases in nature. • 0-100 mol% Al found. >40 mol% Al common in sandy soils. • Co-precipitation, sequential precipitation and agglomeration are expected events in aquatic systems (Anderson et al. 1990). • Properties depends on formation conditions such as metal conc. in solution and pH.

  3. Objectives • General To determine the effect of Al content on P sorption in co-precipitated amorphous Al-Fe hydr(oxide) systems. • Task 1. - To synthesize and characterize the hydr(oxides) based on chemical and physical properties • Task 2. - To determine P sorption characteristics on all hydr(oxides) including the energetics and sorption as a function of time, pH and Al content • Task 3. - Use properties of hydr(oxides) to explain differences in P sorption characteristics

  4. Task 1. Hydr(oxide) Synthesis and Characterization

  5. Synthesis • Al-Fe hydr(oxides) containing 0, 9, 17, 33, 50, 67, 83, 91 and 100 mol% Al. • Co-precipitation from Al and Fe chloride solution at pH 6.5-7.0 by titration with NaOH • Precipitate washed, dried at 70◦C, crushed and sieved (150 μm). • mol% Al used interchangeably with Al:Fe molar ratio.

  6. Characterization • Chemical composition - Metal content ( Al and total) - Acid digestion /atomic absorption • Physical properties - XRD: structure of hydr(oxides) - SEM : morphology and distribution of Al and Fe in particles - PSD (Particle Size Distribution)

  7. Metal Content • Method of synthesis was sufficient. • Total metal content (Al+Fe) decreases linearly with increase Al content. • Due to increase amounts of physisorbed, bound or constitutional water (El-Sharkawy et al., 2000).

  8. XRD • Amorphous, higher intensity area in 10:1(90.9%) more likely associated to Al. • Intermediate transitional stage in crystallization (Carim et al. 1997)Al occupying tetrahedral rather than octahedral position (Wolska et al. 1994 and Rodic et al. 2001)

  9. SEM (morphology) Fe Al 1:10 1:5 10: 1 5:1

  10. Metal distribution Fe Fe Al Al 1:10 1:2 Al Al Fe Fe 2:1 5:1

  11. Metal distribution with depth 1:2 1:1 Al Fe Al @10kV Al @20kV @10kV Al @20kV Fe Fe Fe

  12. 2:1 5:1 @10kV @20kV @10kV @20kV

  13. Al Al Fe • Phase separation • Bayerite (Wolska et. al. 1994) or Corundum (Korecz et. al. 1972) for crystalline Al-Fe oxide. • Korecz suggested a phase change above 5:1 Al:Fe Fe 10:1

  14. Particle Size Distribution

  15. Conclusions • Al-Fe hydr(oxides) formed by precipitation of Al on Fe (>10mol%). Thickness increases with Al content above 50 mol%. • Fe controls the morphology and PSD of hydr(oxide). • Increase structural development with Al content.

  16. Task 2. P-sorption Characteristics

  17. Batch Experiment - P sorption as a function of time , pH, Al content • Flow calorimetry - Heats of P adsorption - Effect of P sorption on surface charge

  18. P sorption by batch • Solid: solution - 1:1000 (10mg/10ml) • P input concentration 100mg/L • Equilibration on reciprocal stirrer for 24hrs except where stated. • P-sorption expressed as - μg/mg hydr(oxide) for time - μg/mg metal otherwise ( to account for the weight contributed by water in the hydr(oxides)).

  19. Al 10:1 1:1 Fe P sorption as a function of time • Increase in sorption with Al content and time. • Fast phase and slow phase thereafter. • Similarity in slope – difference in sorption occurred within 1st hr of equilibration

  20. 90% of 24 hr sorption max occurs within first 3 hrs irrespective of Al content • Rate of adsorption similar , decrease asymptotically towards zero

  21. P sorption as a function of pH • P sorption decrease with increasing pH • Inflection points at 6 and 7 . • No significant decrease between 5 and 7 for 2:1, 5:1 and 10:1 samples may be due to greater structural development.

  22. loss decreases with increasing Al content at fixed pH • pH 3-9: 65-80% for Al and low Al containing hydr(oxides). 50 % for higher Al containing mixed hydr(oxides). • 25-30% loss pH 6-7 for lower Al range and pH 7-9 for higher Al range • Corresponds to pKa values for Fe(6.5) and Al(7.5) (McBride 1994)

  23. Phase separation Complete coverage of Fe by Al Partial coverage of Fe by Al Solid solution P sorption as a function of Al content • Increase with Al content. Al 4*Fe and 2*50-80mol%. • Non-linear, particularly at pH<8 • Each local maxima or minima due to change in property of hydr(oxide)

  24. Conclusions • Al content has no effect on the trend by which P sorption occur nor rate of P sorption. • Al content affects the degree of change in P sorption with increasing pH. Decreasing with increasing Al content. • P sorption increases non-linearly with Al content, particularly below pH 8. • Differences in sorption characteristics largely due to changes in hydr(oxide) properties with Al content.

  25. signal time Flow Calorimetry • Direct, quantitative heat measure (Rhue et al. 2002) • Interaction at liquid/ solid interface • Calorimeter built by Dr. Rhue • Solution + surface sites = signal, which is related to the heat of the reaction. • Heats + quantity sorbed - reaction mechanism + surface properties. thermistor column thermistor

  26. Flow Calorimetry • pH 4.8 • Ion exchange - Cation exchange: K and Ca (50mM and 25mM respectively) - Anion exchange: Cl and NO3 (50mM) - Pre-P and Post-P • P adsorption (1 mM P in 50mM KCl) - 20 minutes

  27. Heats of adsorption Al(100%) 1:1(50%) 1:2(33.3%) 10:1(90.9%) 1:5(16.7%) 5:1(83.3%) 1:10(9.1%) Fe(0%) 2:1(66.7%)

  28. Ion exchange pre-P • NO3/Cl exothermic, Cl/NO3 endothermic and reversible • Reduction in AEC due to P sorption. P sorption and anion exchange occurs on same sites. more likely OH2+ • P sorption is irreversible • Magnitude of change varies with Al content post-P NO3 Cl

  29. Pre-P Post-P • Energy of anion exchange on Al = 4*Fe. Unclear whether this is due to greater energy or more exchange sites. • P sorbed was equal but loss 2*Fe energy units loss on Al. May be due to difference in mechanism of P sorption on Fe vs. Al. • 50% loss in peak area for Fe, 25% for 50-100mol% Al. similarity in loss for indicative of surface similarity.

  30. Conclusion • Al content did not have any effect on P sorption mechanism nor energetics but reduced anion exchange with no apparent change in cation exchange.

  31. Acknowledgement Lord God Almighty My lovely wife, Keisha Dr. R.D. Rhue Dr. W.G. Harris Dr. N. Comerford Dr. V. Nair Bill Reve Keith Hollien Gill Brubaker (PERC) Brad Willenberg (MAIC) My family and friends

  32. Questions ?

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