Effect of Metal-doping of Nanoscale Maghemite on Cr(VI) Adsorption and Nanoparticle Dissolution Jing Hu, Irene M. C. Lo and Guohua Chen Environmental Engineering Program Hong Kong University of Science and Technology Presented at the International Congress of Nanotechnology, October 31-November 3, 2005 San Francisco
Outline Introduction Objectives Methodology Results and Discussions Conclusions
Introduction Introduction Hexavalent chromium, Cr(VI Cr(VI): ): Hexavalent chromium, Highly toxic but valuable Highly toxic but valuable Priority pollutants defined by USEPA Priority pollutants defined by USEPA Electroplating, acid mining, refining, Electroplating, acid mining, refining, petroleum plants petroleum plants
Technologies for heavy metal treatment Chemical precipitation • High equipment costs • Large consumption of reagents • Large volume of sludge • Ineffective recovery of treated metals • Potential hazard to environment Ion exchange • High capital and operating cost • Fouling • Pretreatment Activated carbon adsorption • Large intraparticle diffusion • High regeneration cost • Low regeneration efficiency
Magnetic nanoparticle adsorption Magnetic nanoparticle adsorption Implications for industrial Implications for industrial Advantages Advantages applications applications Comparatively large adsorption Superior removal Comparatively large adsorption Superior removal capacity capacity Very short adsorption time Saved space, especially suitable Very short adsorption time Saved space, especially suitable for crowded cities for crowded cities Easy to separate from treated Easy to separate from treated Lower capital and operating costs Lower capital and operating costs water water Simple to desorb Simple to desorb Easy technical adaptation and Easy technical adaptation and maintenance maintenance No secondary pollution No secondary pollution No potential environmental No potential environmental concern concern
Maghemite nanoparticles for Cr(VI Cr(VI) ) Maghemite nanoparticles for removal removal 20 Am ount adsorbed (mg/g) 15 10 150 mg/L 5 100 mg/L 50 mg/L 0 0 5 10 15 20 25 30 35 40 45 50 55 60 Time (min) Cr(VI) adsorption equilibrium time = 10 min; 50 mg/L of Cr(VI) was reduced to be 0.05 mg/L, below discharge limit
How to enhance adsorption? How to enhance adsorption? 1. Metal-doping technique • Increase Increase in in surface area or active sites surface area or active sites • • S Simple imple m modification method odification method • • Other parameters not impaired Other parameters not impaired significantly significantly, , • e.g., adsorption rate, magnetic properties e.g., adsorption rate, magnetic properties • Stable n Stable nanoparticle anoparticles s • 2. Inorganic coating technique
Objectives Objectives Promotion of adsorption by metal Promotion of adsorption by metal- -doping doping Inhibition of dissolution by metal Inhibition of dissolution by metal- -doping doping Mechanism studies by Raman spectroscopy Mechanism studies by Raman spectroscopy
Materials and Methods Materials and Methods Adsorbent Adsorbent doped γ - Metal Metal-doped -Fe Fe 2 O O 3 nanoparticle (Me= Al, Mg, Cu, Zn, Ni) nanoparticle (Me= Al, Mg, Cu, Zn, Ni) Adsorbate Adsorbate 100 mg/L K 2 CrO 100 mg/L K CrO 4 4 + 0.1 M NaNO + 0.1 M NaNO 3 3 Batch test Batch test Experimental conditions: contact time: 60 min; pH: 2.5; Experimental conditions: contact time: 60 min; pH: 2.5; o C shaking rate: 200 rpm; room temperature: 25 erature: 25 o C shaking rate: 200 rpm; room temp Mechanism study Mechanism study Sample for Raman: 5, 50, 100 mg/L Cr(VI Cr(VI) at pH 2.5, 6.5, 8.5 ) at pH 2.5, 6.5, 8.5 Sample for Raman: 5, 50, 100 mg/L
Analytical Methods Analytical Methods Parameters Analytical methods Cr ICP pH pH Meter Zeta potential ZETA PLUS Particle dimension TEM Particle structure XRD Elemental analysis XRF Complexation Raman spectroscopy Surface area BET Analyzer Magnetism VSM
Raman spectroscopic studies Raman spectroscopic studies Establish symmetry of surface species Establish symmetry of surface species Distinguish inner Distinguish inner- -sphere from outer sphere from outer- -sphere sphere (David et al., 1978; Tejedor Tejedor and Anderson, 1990) and Anderson, 1990) (David et al., 1978; Raman spectroscopic data about PO 3- - , CO 2- - , SeO 2- - , Raman spectroscopic data about PO 4 43 , CO 3 32 , SeO 4 42 , 2- adsorption onto Fe/Al oxides available 2- , and AsO 2 SO 4 , and AsO 4 adsorption onto Fe/Al oxides available SO 4 4 (Schulthess Schulthess and McCarthy, 1990; Su and Suarez, 1998; and McCarthy, 1990; Su and Suarez, 1998; Wijnja Wijnja and Cristian, 2000; Goldberg and Johnston, 2001) and Cristian, 2000; Goldberg and Johnston, 2001) ( Little detailed information on Raman spectroscopic Little detailed information on Raman spectroscopic 2- adsorption onto (modified) iron oxide study of CrO 4 adsorption onto (modified) iron oxide study of CrO
Modification of synthesizing methods Modification of synthesizing methods Sol-gel method Precipitation method Fe 2+ , Fe 3+ , NH 4 OH ( or +Me) Fe 2+ , Fe 3+ , NH 4 OH Surfactant pH 8.0 pH 8.0 (or 10) Magnetite particle Magnetite particle Calcination Oil bath Octyl ether 250 o C oven Maghemite nanogel Maghemite aggregate Ethanol washing Grinding Maghemite nanoparticles Maghemite nanoparticles (> 30 nm, < 80 m 2 /g) (< 20 nm, ~ 250 m 2 /g)
Nanoparticle Synthesis Method (sol- -gel) gel) Nanoparticle Synthesis Method (sol 1.5 M N 2 gas NH 4 OH condenser Thermocouple air 250 o C oil bath Al-doped maghemite ( γ -Fe 2 O 3 ) Al-doped magnetite (Fe 3 O 4 )
doped γ -Fe 2 O 3 TEM images of Al- -doped TEM images of Al Al-doped γ -Fe 2 O 3 with Al-doped γ -Fe 2 O 3 with Undoped γ -Fe 2 O 3 7.5% of Al 13.1% of Al Doping of Al results in preferential crystal growth along [100] direction producing irregular shaped, platy particles, at expense of crystal thickness (Schulze, 1984)
doped γ -Fe 2 O 3 XRD patterns of undoped & Al- -doped XRD patterns of undoped & Al 600 500 Al-doping γ-Fe2O3 γ-Fe2O3 400 Counts 300 200 100 0 10 15 20 25 30 35 40 45 50 55 60 65 70 Degrees 2-Theta A definite proof of structural incorporation can be produced from a shift in position of XRD peaks, but doping would not change original structure
doped γ -Fe 2 O 3 Hysteresis loops of Al- -doped Hysteresis loops of Al 4 γ-Fe2O3 3 7.5% Al-dopant γ-Fe2O3 9.3% Al-dopant γ-Fe2O3 2 11%Al-dopant γ-Fe2O3 Moment (emu) 13.1%Al-dopant γ-Fe2O3 1 0 -1 -2 -3 -4 -10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000 Field (Oe) Magnetic properties decreased with increasing Al dosage
Effect of doped metal on Cr(VI Cr(VI) adsorption ) adsorption Effect of doped metal on 88 86 84 Removal efficiency (%) 82 80 78 76 74 72 70 No- Al- Ni- Cu- Zn- Mg- Metal-dopant maghemite Al-, Cu- and Mg- doping enhanced adsorption capacity; while Cu- and Ni-doping decreased adsorption capacity of previous γ -Fe 2 O 3
Adsorption and separation Adsorption and separation Magnetic Magnetic Surface Adsorption Equilibrium Separation Surface Adsorption Equilibrium Separation Al/(Al+Fe) Al/(Al+Fe) area area efficiency efficiency time time Time Time properties properties (m 2 2 /g) (%) /g) (%) (min) (emu emu) ) (min) (%) (m (%) (min) ( (min) 0 0 162 162 79.8 79.8 10 10 3.48 3.48 0.1 0.1 7.5 182 84.3 25 2.26 0.5 7.5 182 84.3 25 2.26 0.5 9.3 191 86.7 30 1.25 1 9.3 191 86.7 30 1.25 1 9.3 191 86.7 30 1.78 1 11.0 198 87.5 60 1.14 5 11.0 198 87.5 60 1.14 5 13.1 13.1 210 210 88.9 88.9 90 90 / / 10 10
Adsorption mechanism (Raman) Adsorption mechanism (Raman) doped γ -Fe 2 O 3 — Cr(VI Cr(VI) adsorption onto Al ) adsorption onto Al- -doped — 848 882 342 Counts K2CrO4 365 720 498 679 Al-doped γ-Fe2O3 γ-Fe2O3 200 300 400 500 600 700 800 900 1000 1100 1200 Raman shift (cm-1) 2- are all Raman active: the nondegenerate v 1 at 848 Vibrations for the free CrO 4 cm -1 , the doubly degenerate v 2 at 342 cm -1 , the triply degenerate v 3 at 882 cm -1 , and the triply degenerate v 4 at 365 cm -1
Raman spectra Raman spectra — Effect of pH Effect of pH — 100 mg/L Cr(VI) + 5 g/L Al-doped γ -Fe 2 O 3 V1 γ -Fe 2 O 3 at pH 2.5, 6.5, 8.5 831 V3 858 876 V2 NO 3 V4 670 719 359 369 480 338 502 926 719 Counts 1046 670 365 502 840 341 480 863 pH 2.5 932 670 719 502 354 pH 6.5 339 480 848 pH 8.5 200 300 400 500 600 700 800 900 1000 1100 1200 Raman shift (cm-1)
Raman spectra Raman spectra — Effect of surface loading Effect of surface loading — 5, 50, 100 mg/L Cr(VI) + 5 g/L Al-doped V1 831 858 γ -Fe 2 O 3 at pH 2.5 V3 V4 876 V2 670 719 359 369 480 338 502 926 835 Counts 719 868 100 mg/L Cr(VI) 670 502 894 331 366 918 482 50 mg/L Cr(VI) 670 719 867 354 837 480 500 372 331 912 5 mg/L Cr(VI) 200 300 400 500 600 700 800 900 1000 1100 1200 Raman shift (cm-1)
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