HYDROLOGY AND WATER RESOURCES IN ARIZONA AND THE SOUTHWEST, VOLUME... Proceedings of the 1978 meetings of the Arizona Section of...
HYDROLOGY AND WATER RESOURCES IN ARIZONA AND THE SOUTHWEST, VOLUME 8, p. 101 -109. Proceedings of the 1978 meetings of the Arizona Section of the American Water Resources Association and the Hydrology Section of the Arizona Academy of Science, held in Flagstaff, Arizona, April 14 -15. HEAVY METALS & WASTEWATER REUSE Thomas E. Higgins ABSTRACT WasteWater shortages in the Western United States have intensified the search for new sources. One potential impediment water reuse is being increasingly called upon to augment existing supplies. to the continued expansion of wastewater reuse efforts is the accumulation of toxic heavy metals and Conventional and advanced wastewater treatment effect the removal other salts in the recycled water. Removal is by chemical precipitation and adsorption of a portion of the heavy metals added during use. Potential uses of treated wastewater effluents include irrigation and disposed of with the sludges. Care must be taken to prevent contamination of groundwater, especially since and groundwater recharge. existing wells have been reported to have concentrations of heavy metals in excess of drinking water Percolation of wastewaters through soils (espcially fine soils) results in a reduction in standards. It is postulated that removal of these metals is by a combination of heavy metal concentrations. chemical precipitation with filtration of the precipitates, and adsorption on soil particles (thus the effectiveness of fine soils). Long term saturation of the soils with heavy metals may result in a A predictive model of heavy metal -wastewater -soil "breakthrough" contamination of the groundwater. interactions is proposed to aid in the design and regulation of wastewater reuse systems to eliminate or minimize this problem. Introduction Increasing water usage and pollution of surface water supplies is producing a national water This shortage is especially acute in the western states, where sources of surface water are shortage. expensive to develop and /or of questionable quality and groundwater supplies are being depleted. CAP water, Locally, the Central Arizona Project (CAP) was initiated to import Colorado River water. however, is reported to be similar in quality to that of the treated wastewater in the Phoenix area.1 Wastewater reuse is imperative. Other potential water sources are of low quality and /or expensive A portion of Phoenix's wastewater is already committed to be used as to transport to the region. cooling water for the Palo Verde nuclear power plant, presently under construction. Proposed uses for wastewater include irrigation of crops and recharge of groundwater for eventual One potential problem encountered in wastewater reuse is the concentration of toxic heavy metals Drinking water standards for arsenic, barium, cadmium, chromium, lead, mercury, through each use cycle. selenium and silver have been established by the U.S. Environmental Protection Agency.2 Additional standards for copper, iron, manganese, and zinc are being established by the Arizona Department of Health.3 reuse. Table 1. Maximum Contaminant Levels for Inorganic Contamina Level (milligrams Contaminant Arsenic Barium Cadmium Copper Chromium (total) Iron Lead Manganese Mercury Selenium Silver Zinc per liter) U. S. EPA2 Arizona Health Dept.3 0.05 1.0 0.05 1.0 0.01 0.01 -- 1.0 0.05 2.0 0.05 0.05 -- 0.05 0.002 0.20 0.002 0.01 0.01 0.05 0.05 5.0 -- -- The author is Assistant Professor of Engineering, Arizona State University, Tempe. ducted under a grant from the Research Council of Arizona State University. 101 This study was con- Irrigation in arid regions results in the bulk of the water being lost by evapotranspiration. Problems resulting Heavy metals can accumulate in the soils, through chemical precipitation reactions. from an increase in the concentration of heavy metals are inhibition of plant growth and systemic uptake of the metals by crops used for human consumption. In the recharge of groundwaters with wastewater, care must be taken that heavy metal concentraSince evapotranspiration can be minimized, the principal tions do not exceed drinking water standards. heavy metal removal mechanism acting during percolation to the groundwater table would be chemical precipitation and sorption on soils. Existing groundwaters contain heavy metals in concentrations that exceed State and Federal standards (Table 2). This is not to mean that the water supply systems are in violation, but that concentrations of heavy metals in individual wells have been reported to be in excess of the appropriate standards. Table 2. Preliminary Summary of Groundwater Samples Exceeding Drinking Water Standards of Arizona.a Inorganic Contaminant Standardb mg /a No. of samples exceeding standard Max. Mean mg /a mg /e SD Arsenic 0.05 31 1.7 .19 .31 Cadmium 0.01 3 0.1 .045 .047 Chromium 0.05 18 0.20 .10 .04 Lead 0.05 11 1.10 0.32 0.37 Mercury 0.005 15 5.0 1.9 1.7 a. Information supplied by the Arizona Department of Health Services, Bureau of Water Quality Control. These are well samples and thus do not indicate violation of Drinking Water Standards in water systems. b. Minimum values of Federal and State Standards. Wastewaters and Heavy Metals It has been estimated that one municipal usage of water increases the total dissolved solids concentration by 300 mg /z°, with part of the increase due to heavy metals. Wastewaters from urban areas have been found to contain concentrations of heavy metals exceeding drinking water standards.5'6 Davis and Jacknow7 reported that for three cities (New York, Muncie, Indiana and Pittsburgh) the major heavy metal contamination comes from industry (principally electroplating and photoengraving) and this is controllable at the source. A significant portion of the heavy metals is contributed by residential usage and this is not so easily managed by source control. An appreciable portion of heavy metals in domestic wastewater is removed by conventional treatTwo mechanisms are proposed for the removal of heavy metals during treatment. They ment processes. are: 1. 2. Precipitation of metal hydroxides and subsequent removal with the sludge; and Sorption of soluble metals on the sludge. Precipitation of metal hydroxides is increased with increasing pH and therefore increased removal efficiencies would be expected at higher pH's. The precipitation of copper, chromium, nickel, and zinc by sewage has been demonstrated by Jenkins et al.b Increased removal efficiencies were found for increasing initial concentrations, and increasing Removal resulted in decreased pH due to precipitation of hydroxides. pH. Investigating toxicity of chromium to the activated sludge process, Moore, et al.9 showed that up to 0.5 ppm of hexavalent chromium was completely removed by the process. McDermott et al.ln showed that 50 to 79 percent copper removal can be effected by activated sludge over a feed ranging from 0.4 to 25 ppm. McDermott et a1.11,12 found removal of zinc to be between 74 to 95 percent and of nickel to be 30 percent. BarTE et al.13 found that the average efficiencies of activated sludge for removal 102 of hexavalent chromium, copper, nickel and zinc were 44, 75, 28, and 89 percents respectively. Argo and Culp1', showed that tertiary treatment of wastewaters by coagulation, mixed media filtration and carbon absorption were effective at removing cadmium, hexavalent chromium, zinc, and copper and reducing concentrations of other heavy metals. Similar results were found by Linstedt et al. 15 Addition of aluminum or calcium salts for phosphate removal also reduced the concentrations of heavy metals. Because heavy metals are removed in the sludges from wastewater treatment plants they may interfere with the processing of these sludges by anaerobic digestion17, and pose a problem in the use of these sludges for agricultural fertilizers.18 Heavy Metal - Wastewater - Soil Interactions Additional heavy metal removal has been accomplished by percolation of metal containing wastewaters through soi119, suggesting that groundwater recharge could result in removal of heavy metals. Clay soils provided the best treatment. The exchanged metal ions were not leached by water. Movement of heavy metals in coarse soils below sewage disposal ponds was demonstrated by Lund et e1.,20 indicating that wastewater, injected for reuse should be separated from water supplies sources by fine soils. Wang and Nacci21 investigated the movement of lead and copper in two Rhode Island soils. Using lysimeters they measured the gross removal of these metals by the soils, not differentiating between sorption and precipitation. Nelson22 reported on studies in Alabama on the movement of metals used in agricultrue in soils. Laboratory studies indicated a high degree of soil retention of Ca, Ba, Zn, Pb, Na, Cr, Sr and Mg by the six soils tested. Soluble metal --soil equilibrium concentrations followed Langmuir- Freundlich adsorption isotherms. Mechanisms postulated for metal removal by soils included chelation, surface adsorption, precipitation and physical entrapment. Solubility Considerations The solubility of metal salts is calculated from their solubility products. Precipitation or solubility of a metal ion (Mn +) in equilibrium with a counterion (hydroxide in this case) can be expressed by chemical reaction . M(OH)n (s) = Mn+ + n(OH  ) The direction and extent of this reaction can be calculated from the product of the molar concentrations of the species involved in the reaction as expressed in equation . KSP = [Mn +] [OH ]n  Specific solubility equilibria for metal oxides and hydroxides are listed in Table 3. Figure 1 is a plot of the maximum concentration of free metal ions vs. pH. Note that solubility of individual Table 3. Constant for Solubility Equilibria for Heavy Metal Oxides or Hydro Element Reaction KSp Reference Barium Ba(OH)2(s) = Ba2+ + 20H Cadmium Cd(OH)2(s) = Cd2+ + 20H 2.4 x 10-14 24 Copper Cu 0(s) + 2H+ = Cu2+ + H2O 4.5 x 107 24 Chromium Cr (OH)3(s) = Cr3+ + 30H Iron Fe(OH)3 (s) = Fell' + 30H Lead Manganese 5 x 10-3 1 x 10 -30 25 14 2 x 10 -39 24 Pb(OH)2(s) = Pb2+ + 20H 1.6 x 10-15 14 Mn(OH)2(s) = Mn2+ + 20H 2 x 10 -13 24 Mercury Hg0 (s) + H2O = Hg2+ + 20H 3 x Silver Ag20(s) + H2O = 2Ag+ + 20H- 2 x 10-8 14 Zinc ZnO(s) + 2H+ = Zn2+ + H2O 1.5 x 1011 24 103 10-26 14 From this diagram one would expect negligible concentrations metal ions decreases with increasing pH. of Fe3 +, Hg2 , Cr3+ and Cu2+ At neutral pH. Zn2 +, Pb2 +, Cd2 +, Mn2 +, Ag+ and Ba2+ have increasingly greater solubility in equilibria with their oxides and hydroxides. In application of sewage sludges to land it is generally stated that to prevent the solution of metals, the pH of the soil should not be acidic.23 Table 4 lists the pH values needed to maintain the Hydroxides and free metal ion concentration below the Federal and State drinking water standards. oxide precipitation is not sufficient to protect water supplies from Barium, Cadmium, Lead, Manganese, Also total metal concentrations consist of not just the free ion but also include various and Silver. Therefore, total metal concentrations are usually hydroxo metal complexes which must be considered. greater than predicted by this plot pH Below Which Free Ion Will Dissolve to Exceed Standard. Table 4. Element Stam9Rd log Molar Molar Conc. PLI Ba 1.0 7.3 x 10-8 -5.14 15.4 Cd 0.01 8.8 x 10-8 -7.05 10.3 Cu 1.0 1.6 x 10 5 -4.8 6.2 Cr 0.05 9.6 x 10-7 -6.02 6.0 Fe 2.0 3.6 x l05 -4.45 2.6 Pb 0.05 2.4 x 10-7 -6.62 9.9 Mn 0.2 3.6 x 10-6 -5.44 10.37 Hg 0.002 1.0 x 108 -8.00 5.3 Ag 0.05 4.6 x 10' -6.33 16.5 Zn 5.0 7.6 x 105 -4.1 7.6 Figure 2 shows how the formation of soluble Table 5 lists the solubility relationships for copper. hydroxide complexes results in an increased solubility of copper at pH values above neutral. The addition of soluble carbonate species (Figure 3) results in further increase in the solubility of total copper at neutral pH and above. Table 5. Constants of Solubility Eaui ibria for Copper. Log K (25°C) Formula 7.65 CuO(s) + 2H+ = Cu+2 + H20 Cu+2 + 30H 15.2 = Cu(OH)3 Cu+2 + 40H = Cu(OH)42 16.1 Cu2(OH)2CO3(s) + 4H+ = 2Cu+2 + 3H20 + CO 2 14.16 Cu+2 + CO; = CuCO3(ag) 6.77 Cu+2 + 2CO3 = Cu(CO3)2 10.01 Cu+2 + e' = Cu+ 8.8 CuS(s) = Cu + S -36.1 Cu2S (s) = 2Cu+ + S -48.92 Figure 4 shows the Under anaerobic conditions sulfides generally limit the solubility of metals. limited solubility of copper with the total concentration of sulfides of 10'4 Molar. Some metals, whose solubility is large with respect to hydroxides or oxides, may still have limited solubility in natural waters due to the formation of salts of other anions. Two examples are lead carbonate and silver chloride (Table 6). With 10'3 Molar total carbonate species (Figure 5), the soluble lead concentration at pH lis reduced from approximately 33,000 mg /t to approximately 0.13 mg /t. Likewise with 20 mg /s of chloride ion present the soluble silver concentration is reduced from many grams per liter to 0.03 mg /s (Figure 6). 104 o u n Ic' -s - \[cati[e] - IO pH Figure 1. 12 Free Metal Ion Concentrations in Equilibrium with Solid Oxides or Hydroxides. 0 c o ` -6 ó 2 Figure 2. IO pH Figure 3. Solubility of Copper (II) with 10-2 Molar Total Carbonate. 105 4 6 pH 8 10 12 14 Solubility of Copper (II) in Non -Carbonate Waters. pH Solubility of Copper (I) with 10-4 Molar Total Sulfide. Figure 4. 0 3 -7 8 2 4 6 8 10 12 pH Figure 5. o -I 2 -3 -4 -5 Ag CI -6 (CI")= 20mç/1 -7 8 2 4 6 8 I0 12 pH Figure 6. Solubility of Silver with 0 and 20 mg /1 of Chloride. 106 Solubility of Lead (II) with 0 and 10 -3 Molar Total Carbonate. Table 6. Constants of Solubility Equilibria for Lead Carbonate and Silver Chloride.25 Formula KSP (25°C) PbCO3(s) = Pb2+ + co 1.5 x 10-13 AgC1 (s) = Ay+ + Cl- 2.8 x 10-10 Adsorption Heavy metals can further be removed from water by adsorption onto the surface of soil particles. A Langmuir adsorption isotherm describes the equilibrium relationship between the concentrations of ions in solutions and the density of these ions adsorbed on the surface of a solid. Commonly the quantity of adsorbed material increases with increased concentration. A typical Langmuir Adsorption isotherm for Cupric ion and Providence Silt is shown as Figure 7. Langmuir Adsorption isotherm can be linearized by plotting 1 /qe vs. the data for copper is shown as Figure 8. 1 /C. The linearized form of Model A model is proposed to describe the long -term interactions of heavy metals and soils in the reuse of wastewaters for irrigation and groundwater recharge (Figure 9). Heavy metal concentrations in the water at the surface can be determined by mass balance, taking into consideration flow rates of wastewater, precipitation and evapotranspiration and the concentrations of metal in the wastewater. Equilibrium solubility chemistry can be used tu calculate the amount of metal precipitated and filtered in the upper soil and to determine the soluble metal concentration available for adsorption. Downflow through the soil to the groundwater table can be treated as flow through a column. The metal concentration of the goundwater can be calculated by mass balance. The advantage of preparing such a model is that data that can be obtained in a short time on a small sample (eg. adsorption isotherms) can be used to predict the long term effect of the application of heavy metal containing wastewaters to the land. A predictive model of heavy metal -wastewater -soil interactions would be useful in deciding such questions as: 1) 2) 3) 4) 5) Should a particular wastewater be reused? Should heavy metal removal be required of wastewaters in treatment plants prior to reuse? Which soil types are suitable for groundwater recharge by wastewaters containing heavy metals? What is the useful life of a groundwater recharge area? What concentration of heavy metals is expected in fields irrigated with wastewaters containing heavy metals? Conclusions Based on a review of the literature and of solubility and adsorption chemistry the following conclusions can be made concerning the fate of heavy metals when wastewaters are applied to the land for irrigation and groundwater recharge. 1) 2) 3) 4) 5) An appreciable amount of heavy metals are removed in conventional wastewater treatment. Additional removal of heavy metals is effected where advanced wastewater treatment processes (chemical coagulation, sedimentations and filtration) are used. Initial removal of heavy metals in wastewaters applied to land is probably by chemical precipitation and filtration. Additional removal of heavy metals is accomplished by adsorption on soil particles. Fine soils are a better adsorption media than coarse soils. A mathematical model could be prepared using equilibria solubility and adsorption chemistry, groundwater flow theory, and mass balances to predict the long term fate of heavy metals in wastewater applied to the land. REFERENCES CITED 1. Central Arizona Project Environmental Impact Statement, Final, (9/72). Environmental Protection Agency, "Water Programs, National Interim Primary Drinking Water Regulations," Federal Register, 40, (248), 59566, (1975). 2. 107 1.0 -o É 0.8 d.ca 0.6Providence á04;7 02V 00 Figure 7. silt 4g soil -air dried volume-40m1 20 30 40 50 Cu" Concentration, mg /I 10 Adsorption Isotherm for Copper (II) in Contact with Providence Silt (Nelson22). 5 Evopatranagrati w Op Qer Evapotranspiration 4 Mass balance Precipitation - Equilibrium solubility 3 LEEI* Wasawatar zzLm* Water 2 Adsorption Adsorption isotherms -o 0 O 01 02 03 Metal concentrations in groundwater Mass balance I/c (1/mg) Figure 8. Linearized Adsorption Isotherm for Copper (II) in Contact with Providence Silt (after Nelson22). Figure 9. 108 A j Met als A Model of Heavy Metal Movement in Soils. 3. Director of the Department of Health Services, "Maximum Contaminant Levels for Inorganic Chemicals," 4. Bunch, R. L. and Ettinger, M. 5. Barth, E. 6. Mytelka, A. 7. Davis, J. A. and Jacknow, J., "Heavy -Metals in Wastewater in Three Urban Areas," Journal Water Pollution Control Federation, 47 (9), 2292, (1975). 8. Jenkins, S. H. et el., "The Solubility of Heavy Metal Hydroxides in Water, Sewage, and Sewage Sludge -I, the Solubility of Some Metal Hydroxides," International Journal Air and Water Pollution, 8, 537, (1964). 9. Moore, W. A., McDermott, G. N., and Post, M. W. et al., "Effects of Chromium on the Activated Sludge Process," Journal Water Pollution Control Federation, 33, 54, (1971). Proposed 1977 Amendment to Regulation R9 -8 -221. 10. B., "Water Quality Depreciation by One Cycle of Municipal Use," Journal Water Pollution Control Federation, 36, 1411, (1964). F., et al., "Field Survey of Four Municpal Wastewater Treatment Plants Receiving Metallic Wastes," Journal Water Pollution Control Federation, 37, 1101, (1965). I., Czachor, J. S., Guggino, W. B., and Golub, H., "Heavy Metals in Wastewater and Treatment Plant Effluents," Journal Water Pollution Control Federation, 45, 1859, (1973). McDermott, G. N., Moore, W. A., Post, M. A., and Ettinger, M. B., "Effect of Copper on the Activated Sludge Process," Journal Water Pollution Control Federation, 35, 227, (1963). F., Salotto, B. V., and Ettinger, M. B., "Zinc in Relation to Activated Sludge and Anaerobic Digestion Processes," Proc. 17th Ind. Waste Conf. Eng. Ext. Ser. 112, Purdue University, 47 (2), 461, (1973). 11. McDermott, G. N., Barth, E. 12. McDermott, G. N., Post, M. A., Jackson, B. N., and Ettinger, M. B., "Nickel in Relation to Activated Sludge and Anaerobic Sludge Processes," Journal Water Pollution Control Federation, 37, 163, (1965). F. et al., "Summary Report on the Effects of Heavy Metals on the Biological Treatment Processes TT Journal Water Pollution Control Federation, 37, 86, (1965). 13. Barth, E. 14. Argo, D. and Culp, G. 15. Linstedt, K. D., "Trace Element Removals in Advanced Wastewater Treatment," Journal Water Pollution Control Federation, 43, 1507, (1971). 16. Nilsson, R., "Removal of Metals by Chemical Treatment of Municipal Waste -Water," Water Research, 5, 17. Regan, T. M., and Peters, M. M., "Heavy Metals in Digesters; Failure and Cure," Journal Water Pollution Control Federation, 42, 1832, (1970). 18. Webber, J., "Effects of Toxic Metals in Sewage on Crops," Water Pollution Control, 71, 404, (1972). 19. Wentink, G. R., and Etzel, J. E., "Removal of Metal Ions by Soil," Journal Water Pollution Control Federation, 44, 1561, (1972). 20. Lund, L. J., Page, A. L., and Nelson, C. O., "Movement of Heavy -Metals Below Sewage Disposal Pond," Journal Environmental Quality, 5 (3), 330, (1976). 21. Wang, M. C., and Nacci, V. A., Movement of Trace Metals with Percolating Water, Final Report for OWRT, Project Number OWRT -A- 052 -RI. 22. Nelson, W. E., Fate of Trace Metals in Subsoils as Related to the Quality of Ground Water, Final report for OWRT, Project Number OWRR -B -023 -ALA. 23. Council for Agricultural Sciences and Technology, (1976), Application of Sewage Sludge to Cropland: Appraisal of Potential Hazards of the Heavy Metals to Plants and Animals, Office of Water Pro- 24. Stumm, W., and Morgan, J. 25. Latimer, W. M., The Oxidation States of the Elements and Their Potentials in Aqueous Solutions, Prentice Hall, New York (1952). L., "Heavy Metals Removal in Waste -Water Treatment Processes," Water Sewage Works, 119 (8), 62 and (9), 128, (1972). 51, (1971). grams, U.S. EPA -430/9 -76 -013. S., Aquatic Chemistry, Wiley Interscience, New York, (1970). 109
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