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Arsenic Speciation - Year 4/5. Bernine Khan University of Miami Dept. of Civil, Architectural & Environmental Engineering July 9th, 2001. Speciation. Definition: Various species of an element which make up the total concentration of that element
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Arsenic Speciation - Year 4/5 Bernine Khan University of Miami Dept. of Civil, Architectural & Environmental Engineering July 9th, 2001
Speciation Definition: Various species of an element which make up the total concentration of that element • different oxidation states (e.g. arsenic +3, +5, -3) • inorganic - many contain sulfur • organic - contains carbon/hydrogen groups
Why Are We Interested in Speciation? • All soluble arsenic compounds are considered poisonous to humans • Arsenic & its compounds - widely distributed in nature primarily in two oxidation states – As III - arsenite (+3) & As V - arsenate (+5) • As III (+3) - more soluble in water & body fluids & not excreted as readily • Toxicity - a function of magnitude of exposure (concentration accumulated over time.)
Toxicity of Arsenic Species Dependent on chemical form • AsH3 - arsine (gas) – formed under very reduced conditions eg. landfills • As(III) - arsenite • As(V) - arsenate • MMA - monomethylarsonic acid • DMA - dimethylarsinic acid • TMAO - trimethylarsine oxide • AsB - arsenobetaine (marine) * • AsC - arsenocholine (marine) *
Toxicity of Arsenic Species Dependent on chemical form • AsH3 - arsine (gas) • As(III) - arsenite – inorganic (more toxic) • As(V) - arsenate – inorganic - CCA • MMA - monomethylarsonic acid • DMA - dimethylarsinic acid • TMAO - trimethylarsine oxide • AsB - arsenobetaine (marine) * • AsC - arsenocholine (marine) *
Toxicity of Arsenic Species Dependent on chemical form • AsH3 - arsine (gas) • As(III) - arsenite – inorganic • As(V) - arsenate – inorganic • MMA - monomethylarsonic acid • DMA - dimethylarsinic acid • TMAO - trimethylarsine oxide • AsB - arsenobetaine (marine) * • AsC - arsenocholine (marine) * bacteria
Toxicity of Arsenic Species Dependent on chemical form • AsH3 - arsine (gas) • As(III) - arsenite – inorganic • As(V) - arsenate – inorganic • MMA - monomethylarsonic acid • DMA - dimethylarsinic acid • TMAO - trimethylarsine oxide • AsB - arsenobetaine (marine) * • AsC - arsenocholine (marine) *
Arsenic Mobility Eh-pH diagram 0.75 0.50 0.25 0 -0.25 -0.50 -0.75 Oxidizing Condition As(V) +5 As(III) +3 • Measure of system state (O2/no O2) • Soluble As increases with decreasing Eh & increasing pH Eh (volts) AsS +3 As(III) -3 AsH3 (aq) As Reducing Condition 0 2 4 6 8 10 12 14 pH
Arsenic Mobility Eh-pH diagram 0.75 0.50 0.25 0 -0.25 -0.50 -0.75 Oxidising Condition As(V) +5 Most surface waters As(III) +3 Most ground waters Eh (volts) AsS +3 Predicted Landfills As(III) -3 AsH3 (aq) As Reducing Condition 0 2 4 6 8 10 12 14 pH
Field Sampling Methods • Samples collected : • background & detection wells • purged for 20 minutes • Temperature, pH & ORP • stored on ice • refrigerated <4oC • analysed within 48 hrs • for As speciation • analysed for particulate & total As
Current Research Study • Collect and analyse groundwater & leachate samples from MSW & C&D landfill • Three step analysis: • Step 1 – Dissolved phase • Step 2 – Particulate phase • Step 3 - Total phase • Analysis by HPLC-HG-AFS – Speciation ICP-MS – Total Arsenic
Step 1 – Dissolved Phase Sample Sample filtered through 0.45 mm PVDF filter Filtrate (dissolved phase) Analysed for As species by HPLC-HG-AFS* * HPLC-HG-AFS - High Performance Liquid Chromatography- Hydride Generation-Atomic Fluorescence Spectrometry
Speciation by HPLC-HG-AFS • HPLC- Separates the arsenic species • HG- Converts species to a hydride (gas) • AFS – Detects each specie. DL = ~1 mg/L • Only detects hydride-forming arsenic As(III) DMA MMA As(V) 0 5 10 15 Retention time (secs)
Step 2 – Particulate Phase Filter – Particulate phase Microwave digestion with nitric acid (HNO3) (EPA Method 3051) Analysed for Total As by ICP-MS * (EPA Method 6020) * Inductively Coupled Plasma-Mass Spectrometry
Step 3 – Total Arsenic Unfiltered sample Analysed for Total As by ICP-MS Microwave digestion with nitric acid (HNO3) (EPA Method 3051) Analysed for Total As by ICP-MS* (EPA Method 6020) * Inductively Coupled Plasma-Mass Spectrometry
Total & Particulate Arsenic by ICP-MS • Sample dispersed in stream of argon gas • ICP- converts sample to ions • MS – separate ions by to mass & counted • Detection limit = ~0.1 mg/L • Detects all arsenic
GW Speciation ResultsHPLC-HG-AFS MMA (mg/L) As(V) (mg/L) As(III) (mg/L) DMA (mg/L) Total (mg/L) Facility <1 - - <1 - 3.3 <1 - - - - - <1 3.3 <1 <1 - - - - MSW • Most GW samples analysed contained no detectable arsenic
Leachate Results ICP-MS Diss.+Part. AFS Tot. Diss. As(V) As(III) MMA Facility (mg/L) DMA MSW C&D 0.5 - 5.7 4.33 <1 3.65 3.3 1.6 <1 3.3 <1 - - <1 8.2 16.2 13.3 7.8 <1 9.9 3.4 10.2 <1 5.3 - 3.3 <1 - 8.7 16.2 19 12.13 <2 13.55 <9.9 <19.3 <2 8.6 <1 3.3 <1 <1 35.23 43.48 97.24 NA 36.50 39.28 128.89 92.95 6.802 27.53 5.291 5.54 6.782 2.372 - - - - - - <1 6.5 - - - - - - - - - - - - 2.2 <1 - - - - - -
Leachate Results Conc. (mg/L) 140 120 100 80 60 40 20 0 Diss. by HPLC-HG-AFS (readily forming hydride arsenic) Diss.+Part. by ICP-MS (all arsenic) Particulate phase in leachate very small 4 5 8 9 10 11 12 13 14 15 3 1 2 6 7 Landfills
HG-AFS vs ICP-MS all units = mg/L) hydride forming arsenic in diss. all arsenic In diss. all arsenic In part. all arsenic In sample AFS Tot. Diss. ICP-MS Tot. Part. ICP-MS Diss.+Part. ICP-MS Tot. Diss. Facility 41.05 32.92 1.18 1.68 MSW MSW 16.2 13.3 34.33 22.18 + +
HG-AFS vs ICP-MS all units = mg/L) hydride forming arsenic in diss. all arsenic In diss. all arsenic In part. all arsenic In sample AFS Tot. Diss. ICP-MS Tot. Part. ICP-MS Diss.+Part. ICP-MS Tot. Diss. Facility 41.05 32.92 1.18 1.68 MSW 1 MSW 2 16.2 13.3 34.33 22.18 + +
HG-AFS vs ICP-MS 50 40 30 20 10 0 T D-ICP T D-ICP mg/L D-AFS hidden arsenic D-AFS MSW 1 MSW 2 Not all arsenic readily converted to hydrides (non-labile) – not detectable by HG-AFS
Matrix Interference by HPLC-HG-AFS • Good recoverability for As III & As V • Indicates – HPLC-HG-AFS capable of detecting arsenic once it can be converted to a hydride (labile) MMA % As(V) % As(III) % DMA % mg/L spike 7 14 21 90 93 91 104 150 100 129 136 100 96 113 103
Conclusion of Results • Samples from GW and leachate mostly in the inorganic form (As III & AsV) • Inorganic arsenic (As III & As V) are much more toxic than organic arsenic • Speciation of samples show low conc. of arsenic (2-20 mg/L), total analysis show considerable more As present (2-130 mg/L) • Indicates most of the As is tied up (non-labile) & cannot form hydrides easily & therefore are not detectable by HG-AFS • Further analysis is required
Conclusion of Results • Samples from GW and leachate mostly in the inorganic form (As III & AsV) • Inorganic arsenic (As III & As V) are much more toxic than organic arsenic • Speciation of samples show low conc. of arsenic (2-20 mg/L), total analysis show considerable more As present (2-130 mg/L) • Indicates most of the As is tied up (non-labile) & cannot form hydrides easily & therefore are not detectable by HG-AFS • Further analysis is required
Conclusion of Results • Samples from GW and leachate mostly in the inorganic form (As III & AsV) • Inorganic arsenic (As III & As V) are much more toxic than organic arsenic • Speciation of samples show low conc. of arsenic (2-20 mg/L), total analysis show considerable more As present (2-130 mg/L) • Indicates most of the As is tied up (non-labile) & cannot form hydrides easily & therefore are not detectable by HG-AFS • Further analysis is required
Conclusion of Results • Samples from GW and leachate mostly in the inorganic form (As III & AsV) • Inorganic arsenic (As III & As V) are much more toxic than organic arsenic • Speciation of samples show low conc. of arsenic (2-20 mg/L), total analysis show considerable more As present (2-130 mg/L) • Indicates most of the As is tied up (non-labile) & cannot form hydrides easily & therefore are not detectable by HG-AFS • Further analysis is required
Conclusion of Results • Samples from GW and leachate mostly in the inorganic form (As III & AsV) • Inorganic arsenic (As III & As V) are much more toxic than organic arsenic • Speciation of samples show low conc. of arsenic (2-20 mg/L), total analysis show considerable more As present (2-130 mg/L) • Indicates most of the As is tied up (non-labile) & cannot form hydrides easily & therefore are not detectable by HG-AFS • Further analysis is required
Current Work • Environmental Experiment: Continue quantification of arsenic species from C&D landfills (Florida): • Groundwater samples • Leachate samples • Laboratory Experiment: Analyze leachate from lysimeters designed to simulate C&D landfill conditions – filled with treated and untreated wood waste • Subject unburned wood & wood ash to the TCLP & SPLP
Current Work • Environmental Experiment: Continue quantification of arsenic species from C&D landfills (Florida): • Groundwater samples • Leachate samples • Laboratory Experiment:Analyze leachate from lysimeters designed to simulate C&D landfill conditions – filled with treated and untreated wood waste • Subject unburned wood & wood ash to the TCLP & SPLP
Current Work • Environmental Experiment: Continue quantification of arsenic species from C&D landfills (Florida): • Groundwater samples • Leachate samples • Laboratory Experiment: Analyze leachate from lysimeters designed to simulate C&D landfill conditions – filled with treated and untreated wood waste • Subject unburned wood & wood ash to the TCLP & SPLP
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