The Application of CaSO4 Soil Amendment to the Retardance of ...
Retardance of RainwaterLeached Metals in Amended Soil Systems: A Case Study John D. Harden, P.G., Ph.D. Robert E. Pitt, P.E., Ph.D., BCEE, D.WRE Department of Civil, Construction & Environmental Engineering The University of Alabama Tuscaloosa, Alabama Presentation Overview Goal and Scope of Research Background Ash and Soil Classification Batch Studies Column Studies Conclusions Future Work Research Goal Identify a readily available agricultural soil amendment to reduce the mobility of metals at Chromated Copper Arsenate (CCA)-treated wood burn sites. Iron sulfate, agricultural lime, and gypsum were studied. The results for gypsum (CaSO42H2O) are presented. Scope of Research What are the potential environmental
effects on soil and water resulting from the burning of CCA-wood? What is the composition of the ash? What is the fate of the ash in soil? How effectively are metals leached from the soil/ash by rainwater? Can the leaching rates be reduced by the addition of soil amendments? Background What is CCA? Chromated Copper Arsenate (CCA) wood preservative Designed for the humid Southeastern U.S. where wood is prone to insect and fungal attack Cu - serves as a fungicide As - serves as an insecticide Cr - a fixing agent for bonding Cu & As to wood Used in residential-use wood until 2004
Very large amounts of wood were treated Pictures: www.google.com/search?q=pictures+of+CCA+wood Statistics on CCA Production ~1lb CCA preservative/3 ft3 treated wood 1997: 450 x 106 ft3 of CCA-wood products Service life of CCA-wood 20-50 years CCA-treated wood waste will increase from 5 to 32 x 106 ft3/yr by 2015 As in CCA-wood waste: 31,000 metric tons introduced into Florida environment over the past 30 years Burning of waste/old CCA-treated wood has been common practice
Name Symbol Copper Cu Chromiu m Cr Arsenic As Metal Species in CCA-Wood/Ash Oxidatio n State Cu(II) Cr(III) Cr(VI) As(III) As(V) Cation/ Anion Cation Anion
Anion Anion Anion Species Characteristics Cu+2 CuOH+ Cu2(OH)4- varying toxicity varying solubility varying mobility Cr Hydroxides Cr(OH)4Chromate CrO4-2 Dichromate Cr2O7-2 Arsenite AsO3-3 Arsenate AsO4-3 less toxic less soluble less mobile more toxic more soluble more mobile
more toxic (25-60 times) more soluble more mobile less toxic less soluble less mobile Investigations to Quantify Metals and Evaluate Mobility Produce ash & Quantify metals in CCA-ash How mobile are the metals? Batch Leaching Experiments Accelerated leach studies Regulatory classification of CCA-ash Leaching of CCA-ash & a soil/CCA-ash mixture Potential contamination of water by CCA-metals Effect of CaSO4 on metals mobility Optimization Study Column Leaching Experiments Evaluate CaSO4 performance versus natural soil Measure the pH range of the leach events Sampling Points & CCAmetals Concentration at
Burn Site Upgradient CCA-metals at Burn Site As (12.21.2) Cr (17.41.4) Cu (14.50.5) Sampling: 200 (baseline) & 10 mg g-1 site soil 95% CI Burn Site Burn Site Lake Tuscaloosa Alabama Down-gradient 10 & 16 Sample points Scale 1ft Soil & CCA-ash Soils in US & Proportions of Major Adsorbed Cations on Surface Layers Soil Order
Typical location H+ and Al+3 Ca+2 Oxisols Hawaii 85 10 3 2 trace Spodosols New England 80 15 3 2 trace Ultisols
Southeast U.S. 65 25 6 3 1 Alfisols PA to WI 45 35 13 5 2 Vertisols AL to TX 40 38 15
5 2 Mollisols Midwest U.S. 30 43 18 6 3 Aridisols Southwest U.S. 65 20 10 5 Adapted from Brady, 1990 Mg+2 K+
Na+ Total CCA-metals in 1g CCAash 1000mg CCA-ash contains 232mg (23%) CCAmetals CCA-metals As, 70mg 30% As Cu Cu, 86mg 37% Cr Cr, 76mg 33% Microwave digestion & ICP Batch Leaching Establish preliminary leaching trends for column design Metals Present and Leached from Soil/Ash Mixture (mg g-1 & %) 1000 100 232 Log Metals (mg g
-1 Ash) Total Metal (mg) Leached Metal (mg) Cu Cr As 86 76 70 10 6.9 4.2 1 2.6 0.1 0.01 0.03 1
3.0% 3 0.03% 5.5% Mass % Leached 2 4 Leaching Trend : Cr>As>Cu 3.7% Regulatory Classification of CCA-ash by RCRA-TCLP As - Regulated Haz. Waste (RCRA) TCLP Limit: 5.0 mg L-1 CCA-ash by TCLP: 57844.7 mg L-1 Cr - Regulated Haz. Waste (RCRA) TCLP Limit Cr(VI): 5.0 mg L-1 CCA-ash Cr(VI+III) by TCLP: 0.7190.057 mg L-1 Cu - Not regulated under RCRA CCA-ash Cu by TCLP:
6.720.713 mg L-1 Results reported in mg L-1 95% CI Therefore CCA-ash is classified as a regulated RCRA Haz. Waste due to As -1 Ash 100.0 100 Log Leached Metal mg g 10 10.0 Batch Leaching of CCA-ash & Soil/CCA-ash Mixture Cr As 17.1 4.23 10.1 2.60 1.0 1.0
1150% Decrease 1550% Increase 3 Ash-Soil Releach Therefore natural soil retards Cr & As mobility Potential Contamination of Water by CCA-metals Batch Leach (5 Leachings) 1.45 g CCA-ash, 8.25 g Test Soil, 465 mL Total Rainwater Metal Leached (g g-1ash) Regulatory Level (g L1 ) Cu Cr 1130 2900 10* 100** 113*** 29***
As 4700 10** 469*** L g-1 ash * Cu based on toxicity to freshwater clams, Harrison, et al. 1984 ** SDWA-MCL *** Volume of Water Contaminated to Regulatory Level Leached CCA-metal (mg-1 gCCA-ash ) ash) CaSO4 Optimization Batch Study 4 3 3.21 3.11 2.83 2 2.53 Cu Cr
As 1.77 1.90 1.57 1.24 1 1:1 0.023 0.047 0 -50 0 4:1 0.047 16:1 0.164 50 100 150 200 250 300 350 400 450 Gypsum Addition (mg) A 1:1 to 3:1 ratio of CaSO4:CCA-ash was Column Leaching Objectives Evaluation of CaSO4 performance
Accelerated leaching equivalent to annual rainfall at the burn site - 19 Rainfall events = 130 cm yr-1 Evaluation of metals leached mass determined for each rainfall event - 49 g CCA-ash, 68 g CaSO4, 231 g soil - 1.6:1 Ratio of Ca+2 to CCA-metals Measure leachate pH for each rainfall event - pH Range: 7.3 to 8.0 Mobility Reducing Mechanisms Outer-Sphere Complex -- Diffuse Ion - - - - - - - - - - - - Inner-Sphere Complex -------------------- Negative Surface Charge on Soil Particle Soil/CCA-ash/CaSO4 System
Ultisol Soil Low Cation Exchange Capacity (CEC), 35% efficiency Low to moderate Anion Exchange Capacity (AEC) CCA-ash XO => X(OH)2 => XCO3 , where X = Ca, Mg, Cu, Cr, As General Mechanisms from CaSO4 Amendment Ca+2 replaces Al+3, increases CEC efficiency Increased Al(OH)3 and Cr(OH)3 precipitation Increased Ca+2 concentration and negative surface charge produces greater adsorption and coprecipitation of As and Cr. Increased Cu mobility due to competition with Ca+2 for adsorption sites and increased mobility of Cu-bound organic compounds Accelerated One-Year Mass Leach of Metals Control Cr CaSO4 2.53 0.578 Control 4.31
1.4 Cr 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400% CaSO4 reduces mobility of As by a further 77% and Cr by 72% over soil alone Leached Cu mg g-1 Ash Cu 0.012 0 1.8 0 0.030 Control CaSO4
As 0.15 2.0 Ash Control Mass (mg g-1) -1 As Combination Leached Cr mg g Meta l 0.20 Leached As mg g Accelerated One-Year Mass Leach Control vs CaSO4 0.25 -1 0.30
500 1000 1500 Cumulative Leach Volume (mL) 0.025 Control CaSO4 0.020 Cu 0.015 0.010 0.005 0.000 0 500 1000 1500 Cumulative Leach Volume (mL) Cu mobility increases by a small mass compared to the large reduction 2000
2000 Conclusions Unamended soil retards leaching of As & Cr from CCA-ash while increasing Cu mobility CaSO4 amendment of soil further reduces mobility of As by 77% and Cr by 72% Optimization study revealed 3:1 ratio of CaSO4 to CCA-ash mass is recommended A higher ratio of CaSO4 to CCA-ash mass would serve as a continued source of Ca+2 cations for long-term stabilization of As & Cr CaSO4/Soil/CCA-ash system: 7.3 to 8.0 pH Ongoing Work & Future Potential Soil and Sediment Contamination journal has published two articles from this research on CCA- metals adsorption/desorption
mechanisms in amended soil systems and a third article is under review in another journal The future of metals immobilization in soil by the use of common soil amendments could involve the manufacture of enhanced soil amendments to improve the performance of immobilization mechanisms Presentations Parts of the research results have been presented at the following: EWRI-World Environmental & Water Resources Congress 2008 Honolulu, HI American Water Resources Association 2010 Annual Conference Philadelphia, PA Questions ? [email protected]
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