Sorption-Desorption at Colloid-Water Interface: A Phenomenon of Environmental Significance Soil Chemical Processes and Ecosystem Health Soil is THE MOST IMPORTANT sink of contaminants Calculated equilibrium distribution of contaminants (Crosby, 1982) Distribution (%) Uttam Saha 1 Air Water Sediment Soil 0.35 0.05 9.1 90.5 Soil s Resilency Soil s resiliency to chemical stress: its unique ability to accept, hold, and release contaminants -Chemical properties of the contaminants -Chemical properties of soil (colloids: clay and OM) -Chemistry and biochemistry of the surrounding Soil chemical reactions: precipitation/dissolution adsorption/desorption complexation/dissociation immobilization/mineralization oxidation/reduction catalysis 3 Interface Two Phases Coexist: The tiny zone/common boundary in between the two Properties influenced by both Examples: Solid-liquid Liquid-Gas Liquid-liquid (Oil-water) 4 4 Sorption, Adsorption, and Absorption Surface Functional Groups and Surface Complexation Intra-particle penetration e.g., Isomorphous substitution Adsorbate an Fe Al 5 Si 6 5 1
Adsorption: Types of Surface Complexes Hydrated Cl - -ve polarity HPO 4 2- +ve polarity Hydrated Ba 2+ Hydrated Pb 2+ Vt-K + forms Inner-sphere; Why? Mt-Ca 2+ or K + forms Outer-sphere; Why? Vt-Ca 2+ forms Outer-sphere; Why? 7 8 Adsorption Isotherm At constant T, ph, and I, Sposito (1984): The Surface Chemistry of Soil Metal on Var. Chrg Miner. Cu-Ads. On Soil: Courtsey of C.S. LeVesque P-ads. On Soil: J.Soil Sci. 25:242, 1974 ph for sharp increase Cd-Ads on Soil: WASP 9:289, 1978 Parathion- Ads on Soil: SSSAJ 36:583, 1972 Sorption Affinity/selectivity Pb>Cu>Zn>Co Ni>Mn 9 Ads. Maxima; Ads. Affinity; Types Reactive Sites 10 Desorption: The Reverse Process Pb Desorption [H + ] increases Pb Desorption Arsenate Adsorption on Various Adsorbents 11 12 2
OUTLINE Competitive Adsorption of Arsenate and Phosphate on Some Phyllosilicates, Few Variable-Charge Soils, and Various Synthetic Metal Oxides Competitive Adsorption of Arsenate and Phosphate on Some Selected Organo-Mineral Complexes Adsorption of Arsenate on Some Selected Metal Oxides and a Soil in the Presence of Some Common Rhizosperic Organic Acids 13 Natural geological processes Anthropogenic sources Sample Uncontaminated soils Contaminated soils Introduction Arsenic (As)-an ubiquitous toxic metalloid in soil/water and sediment/water environments Total As < 5 ppm (Raven et al., 1998) 5 to 2553 ppm (Walsh and Keeny, 1975) Fresh waters 1 to 3000 ppb (Westcot et al., 1993) Its toxicity to humans, animals and plants are well 14 documented BANGLADESH Map of drinking water As level Up to 200 ppb Up to 50 mg/kg 1.4 million kg/yr Increasing @ 1mg/kg/yr Current As Problem in Bangladesh Agriculture Food Safety As-contaminated drinking water 40% of the area 15 Phytoxic Effects Food Production Food Security 16 USEPA has lowered the MCL of total As in drinking water from 50 to 10 ppb (USEPA, 2001) Arsenic occurs in two oxidation states As(III) and As (V) As(III) is more mobile and toxic than As(V) Because the kinetics of As redox transformations are slow, both As(III) and As(V) are often found in soils regardless of redox conditions 17 Sorption of As by soil constituents largely controls the mobility and bioavailability of arsenic in soilwater-plant systems Leaching of As poses one of the potential risks to groundwater quality 18 3
Phosphate is an important inorganic ligand in the soil-water and sediment-water environments Phosphate h in solution Compete with Arsenate for adsorption Desorb the adsorbed Arsenate Impact on As in solution As Bioavailability LMMOLs present in the rhizosphere may have an important role on the mobility of As at root-soil 19 interface The role of soil components and solution cheistry on arsenate adsorption under different ionic environments is poorly understood Therefore, As Sorption Studies were carried out on : Various metal oxides Phyllosilicate clays Natural soils, and Organo-mineral comple xes ph Phosphate and organic ligands Surface coverage and contact time 20 STUDY-1 Sorption of Arsenate and Phosphate from Single Oxyanion Systems Generate Sorption Isotherms Determine Max. Sorption Capacity 21 Surface Area (m 2 /g) and Point of Zero Charge (PZC) of Metal Oxides Oxide Surface Area PZC 1. Goethite 85 7.8 2. Ferrihydrite 200 7.6 3. Gibbsite 120 89 8.9 4. Boehmite 460 8.5 5. Birnessite 40 2.5 6. Pyrolusite (Aldrich) 44 2.7 22 Other Sorbents Studied 7. Kaolinite (Georgia; KGa-1) (BET-N 2 SSA = 9.1 m 2 /g) 8. Montmorillonite (Wyoming; SWy-1) (BET-N 2 SSA= 18.6 m 2 /g) 9. Nontronite (BET-N 2 SSA = 16.0 m 2 /g) 10. Allophane (Al-Si Sample provided by Dr. Mora; Mora and Canales, 1995) (BET-N 2 SSA = 717 m 2 /g) 11. Andisol-16 (C horizon of a Typic Hapludand from Roccamonfina Volacano, Italy; provided by Dr. Violante) (42% allophane) 12. Oxisol from China (Provided by Dr. Violante; He et al., 1998) (Mineralogy: KAOL., ILL., Vt+HIV) 23 Sorption Maxima of As and P (mmole/kg) (the anions were added alone) Sorbents ph 4.0 ph 7.0 1. Kaolinite 2. Montmorillonite 3. Nontronite 4. Allophane 5. Oxisol 6. Andisol-16 7. Gibbsite 8. Boehmite 9. Goethite 10. Ferrihydrite 11. Birnessite 12. Pyrolusite PO 4 AsO 4 PO 4 AsO 4 20.8 18.0 12.7 8.1 14.7 12.5 11.4 8.5 5.7 6.3 5.6 5.8 1318 72.6 822 318 555 160 608 28.6 24.2 1218 54.1 738 300 486 193 662 29.9 27.2 188 60.8 644 225 315 136 355 15.8 18.7 113 45 452 151 292 152 399 17.3 23.2 24 4
ph effects on As and P adsorption s adsorbed (mmole/kg) Anions 500 400 300 200 Andisol 16 100 As P Gibbsite 0 3 4 5 6 7 8 9 ph Predetermined quantity of 0.01 M As or P was added alone to yield nearly 100% surface coverage of each ligand for gibbsite and 50% surface coverage for Andisol, as previously determined by 25 adsorption isotherm ph effects on As and P adsorption adsorbed (mmole/kg) Anions a 200 180 160 140 120 100 80 60 40 20 0 As P A Goethite Pyrolusite 3 4 5 6 7 8 9 ph Predetermined quantity of 0.01 M As or P was added alone to yield nearly maximum adsorption of each ligand, as previously determined by adsorption isotherm 26 Summary In the single ligand system, For a given sorbent, maximum adsorption capacities for phosphate and arsenate were often similar ph had negative effect on both phosphate and arsenate sorption STUDY-2 Competitive Sorption of Arsenate and Phosphate Sorption from Binary Systems of As and P 27 28 Sorption (mmol/kg) of P and As, when added as a mixture (As/P = 1) (added amount was to achieve max. surface coverage on selected sorbents at ph 4.0) Rf = As sorbed/p sorbed molar ratio Sample P sorbed As sorbed Rf Birnessite 16.0 26.7 1.73 Goethite 79.0 92.0 116 1.16 Andisol-16 762 410 0.54 Gibbsite 277 130 0.45 Kaolinite Montmorillonite 19.6 11.1 6.4 9.2 0.32 0.82 29 Molar ratio of As/P adsorbed (Rf) Evaluate the Sorption Preferences (when As and P were added as a mixture with different As/P MRs) 2.50 2.00 1.50 1.00 0.50 0.00 0.27 0.62 Gibbsite Ferrihydrite 0.53 105 1.05 0.69 0.50 1.00 2.00 1.98 Initial As/P molar ratio in solution (Ri) Rf>Ri: As Prefd. over P Rf=Ri: None Preferred Rf<Ri: P Prefd. over As 30 5
Summary In general, the Rf values for the sorbents increased in the order of: Allophane < gibbsite ~ noncrystalline Al(OH)x < kaolinite < boehmite < illite ~ montmorillonite < nontronite < goethite ~ ferrihydrite < pyrolusite <birnessite. Note: Higher Rf value means As was preferred over P for sorption STUDY-3 Competitive Sorption of Aresenate and Phosphate on Mixed Fe-Al Oxides To confirm: P is preferred over As on Al-rich sorbents As is preferred over P on Fe- & Mn-rich sorbents 31 32 Synthesis Method for Fe-Al Mixed Metal Oxides Samples preparation 0.1 M Al(NO 3 ) 3 + 0.1 M Fe(NO 3 ) 3 Fe/Al Molar Ratio or Ri = 0, 1, 2, 4, 10 or Slow Titration with 0.5 M NaOH up to ph 5.5 (ph 7.0 for R 0 0) XRD of the samples aged 7 days at room temperature. F=Ferihydryte G=Gibbsite Mixed Fe Al oxides aged for 7 d at r.t. (1 d for R0) Chemical analysis Reactivity Mineralogy -chemical composition Competitive Sorption of -charge characteristics -XRD, IR, TEM As and P on Metal - surface area Oxides 33 Note: R 0 means pure Al(hydr)-Ox; R means pure Fe(hydr)-Ox Non-cryst. Al-prec.+ Some para-cryst. Gibbsite 34 R0, R1, R2, R4, R10, and R(infinity) represent different e initial Fe/Al molar ratios TEMs of the mixed Fe-Al oxides R0 R2 R10 R1 R4 R Para-cryst. Gibbsite 35 Characteristics of the synthesized mixed Fe-Al oxides. Sample %(Fe+Al) solubilized by oxalate Fe ox /Al ox molar ratio (R ox ) BET N 2 - Surface area m 2 /g Mineralogy R0 20 0 230 Poorly crystalline Al(OH)x + Gibbsite R1 82 1.3 275 6-line ferrihydrite R2 85 2.3 275 6-line ferrihydrite R4 93 4.6 280 6-line ferrihydrite R10 92 9.8 285 6-line ferrihydrite R 74 195 6-line ferrihydrite Notes: Acid Oxalate selectively extracts non-cryst. Fe and Al Fe ox /Al ox = the ratio of oxalate extractable Fe to Al 36 6
Sorption (mmol/kg) of As and P added as a mixture (As/P = 1) at ph 5.0 Samples Ri (Fe/Al) As sorbed P sorbed Rf R0 R1 0 1 384 426 855 830 0.45 0.51 R2 2 503 753 0.67 R4 4 610 769 0.79 R10 10 537 546 0.98 R 508 427 1.19 Note: added amount of As was enough to achieve max surface coverage Rf = As sorbed/p sorbed molar ratio 37 Rf values at ph 4.0 and 7.0 (1/Rf values are in the parentheses) As and P were added as a mixture (As/P = 1) Sorbent ph 4.0 ph 7.0 Pyrolusite 1.31 (0.76) 1.25 (0.80) Goethite 116(086) 1.16 (0.86) 100(100) 1.00 (1.00) Gibbsite 0.45 (2.22) 0.37 (2.70) Andisol-16 0.54 (1.86) 0.30 (3.33) Note: added amount was to achieve max surface coverage Rf = As sorbed/p sorbed molar ratio 1/Rf = P sorbed/as sorbed molar ratio 38 Summary Fe- and Mn-oxides were more effective in adsorbing As than P On the contrary, minerals rich in Al (gibbsite, noncryst-al(oh)x and allophane) were much more effective in adsorbing P than As Competitiveness between P and As changed at different ph values The Rf (sorbed As/sorbed P) values decreased by increasing the ph 39 Summary (Contd.) The 1/Rf (sorbed P/sorbed As) values increased by increasing ph P inhibits As sorption on the sorbents more in neutral and alkaline systems than in acidic systems As inhibits P sorption on the sorbents more in acidic systems than in neutral and alkaline systems 40 Let s Have a Break! 41 7