Reactions of Copper Sulfate with Wetland-Taro Soils in Hawaii

Transcription

1 COMMUN. SOIL SCI. PLANT ANAL., 28(11&12), (1997) Reactions of Copper Sulfate with Wetland-Taro Soils in Hawaii N. V. Hue, Fengmao Guo, Guoqing Zhang, R. S. Yost, and S. C. Miyasaka Department of Agronomy and Soil Science, College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, Honolulu, HI ABSTRACT The environmental impact of copper sulfate (CuSO 4 5H 2 O) must be evaluated before the chemical can be registered as a pesticide to control the apple snail (Pomacea canaliculata) in Hawaii's wetlands. To help achieve this goal, we investigated the sorption-desorption reactions of CuSO 4 5H 2 O with six wetland-taro soils (Tropaquepts) of Hawaii. Our results indicated that: (i) copper (Cu) was sorbed rapidly: % of the added Cu was removed from solution within one hour when the loading rate was less than or equal to 300 mg Cu kg -1 [initial Cu concentration = 30.0 mg Cu L -1 or 12 kg (ha-cm) -1 as CuSO 4 5H 2 O which is 10 times the maximum recommended rate of pesticide applications, (ii) Cu sorption increased as soil ph increased from 5.0 to 8.0, and (iii) sorption capacity varied from 210 mg Cu kg -1 in a Tropaquept from Kauai Island to 500 mg Cu kg -1 in another Tropaquept from Maui Island, after seven days of incubation at soil-solution ph 6.0 and total solution Cu concentration of 0.10 mg Cu L -1, a Cu level deemed toxic to some living organisms. It appears that more Cu was sorbed (less Cu remained in solution) if the soil contained high organic carbon (C) and low indigenous 849 Copyright 1997 by Marcel Dekker, Inc.

2 850 HUEETAL. Cu. Also, there was an inverse relationship between Cu sorption and desorption by the soils tested: the more Cu a soil can sorb, the tighter it holds Cu, and the less Cu it releases. Since soil ph increases by 1 to 1.5 units upon flooding and Cu sorption increases with increasing ph, the recommended practice of flooding the soil for at least 48 hours between CuSO 4 5H 2 O application and crop planting should be followed. INTRODUCTION In Hawaii, CuSCy5H 2 O has been used as a pesticide to control the apple snails which are a serious pest for wetland crops, such as taro (Colocasia esculenta) and water cress (Nasturtium officinale). The maximum recommended rate is 1.2 kg CuSCy5H 2 O ha-cnv 1 of water (2.7 lbacre-inclr 1 ), corresponding to an initial concentration of 3.0 mg Cu I/ 1. Water must be ponded for at least hours and no more than four applications of CuSO 4-5H 2 O may be applied to one taro crop at intervals of two months (proposed pesticide label; M. Isherwood, personal communication). However, along with the pesticidal effects on snails, high levels of Cu in water are toxic to many non-target organisms, ranging from fish to plants (Cheng, 1979). For example, the median lethal concentration (LC 50 ) for the embryo and larvae of rainbowtroutiso.il mgcul/ 1 (Birge and Black, 1979). Levels higher than mg L' 1 ( \im of total Cu in solution can damage or kill the roots of many crop plants (Baker, 1990). Thus, proper use of CuSCy5H 2 O requires a good understanding of Cu reactions with soils and a reliable prediction of Cu concentration that remains in water hours after the application of CuSCy5H 2 O. In soils, Cu is adsorbed on the surface of clay minerals, Fe and Mn oxides, and organic matter; it can precipitate with sulfide, carbonate, and hydroxide ions and complex with soluble organic molecules (McBride, 1981; Baker, 1990). Soil acidification reportedly enhances Cu desorption from the solid phase, thereby increasing its concentration in the soil solution (Msaky and Calvet, 1990; Basta and Tabatabai, 1992). This enhanced desorption may intensify the bioavailability/ toxicity of Cu. Specifically, Cu uptake by plants and animals is a function of the activity of soil-solution Cu 2+ which is controlled by the concentration of total soluble Cu and inorganic and organic ligands and by soil ph. A decrease in ph enhances Cu 2+ absorption by living organisms because of increased solution activities of Cu 2+ associated with the dissolution of soil minerals, reduction of organic complexation, and reduction in solid-phase adsorption of Cu (Baker, 1990). The objectives of this study were (i) to determine soil properties that control Cu sorption/desorption, (ii) to characterize the time-dependent property of Cu sorption, and (iii) to quantify the relationship between added CuSCy5H 2 O and Cu in the soil solution.

3 REACTIONS OF COPPER SULFATE 851 MATERIALS AND METHODS Soil Description and Laboratory Analysis Six soils of the Great Group Tropaquept, representing major wetland taro growing areas in Hawaii, were used in this study. Two of the soils were the Hanalei series, which were collected from the Hanalei valley (denoted Taro Hanalei) and Kapaa (Rice Hanalei) on Kauai Island; two were the Pearl Harbor series (Pearl Harbor-1 and Pearl Harbor-2), which were collected from Kahaluu on Oahu Island; the last two soils, as yet unnamed, were from Keanae, Maui Island (Maui soil), and the Waipio valley, Hawaii Island (Waipio soil). The soils were air dried, crushed, and screened to pass a 0.84-mm (20-mesh) sieve for chemical analysis and a 0.25-mm (60-mesh) sieve for x-ray diffraction analysis. The x-ray diffraction was performed with a cobalt Ka x-ray source set at 40 kv and 20 ma. Scanning step was 0.05 degree 29, and scanning angles were from 4 to 76 degree 20. Soil organic C was determined by the Walkley-Black method (Nelson and Sommers, 1982) and clay content by the pipet method (Gee and Bauder, 1986). Non-crystalline iron (Fe) and aluminum (Al) contents of the soils were extracted with 0.2Mammonium oxalate solution (1:80 soil/solution, 2-hr shaking). Soil Cu in the original samples was extracted with AM hydrochloric acid (HC1) (1:4 soil/ solution, 30 min shaking), 0.1MHC1 (1:10 soil/solution, 5 min shaking), and 5mMDTPA-TEA (1:2 soil/solution, 2 hr shaking). The Cu-extracting procedures were similar to those described by Baker and Amacher (1982). Soil ph was measured on the original soil samples [1:1 in water and 1:10 in 5 mm calcium chloride (CaCl 2 )] and after 48 hrs of submergence (1:10 soil/solution in 5 mm CaCl 2 ). Time-Dependent Experiment on Copper Sorption Duplicate samples of 1.00 g soil were suspended in 10 ml 300 mg Cu L" 1 and the suspension was shaken for 5,10,30,60, or 180 minutes at rpm, then centrifuged for five minutes at 12,000 g. Copper in the supernatant was measured by atomic absorption spectrophotometry (AA). The experiment was repeated with a lower loading rate of 300 mg Cu kg" 1. Additionally, solution Cu concentrations were measured after one hour of equilibration for loading rates of 750andl,500mgCukg-'. Copper Sorption Isotherms Copper sorption isotherms at ph 5.0, 6.0, 7.0, and 8.0 were established for each soil as follows: 2.00 g of soil were suspended in 40 ml of 5 mm CaCl 2 containing either 0, 100, 200, 400, 800, 1,200, or 2,000 mg Cu kg 1 soil as CuSO 4-5H 2 O. Each treatment was duplicated. The target ph was achieved by adding 0.01MHC1 or sodium hydroxide (NaOH) 48 hrs after Cu addition. The

4 TABLE 1. Soil properties that may affect Cu reactions in six wetland-taro growing soils of Hawaii. Soil SoilpH* Organic Clay Oxalate-ext. Extractable Cu (Tropaquept) BF AF carbon fraction Al Fe 4/WHCI 0.1MHCI DTPA -- --».-^ ^»»»s* «a** niy vu ^y -«^» RiceHanalei 4.42(4.55) TaroHanalei 5.03(5.28) Maui 5.06(5.36) Pearl Harbor (5.51) Pearl Harbor (5.18) Waipio 5.50(6.11) f BF: before flooding (values in parentheses are soil ph in 1:1 water); AF: after flooding for 48 hours (1:10 in 5 mm CaCl 2 ).

5 REACTIONS OF COPPER SULFATE 853 soil suspension was equilibrated for a total of seven days before centrifugation and filtering. Copper in the filtrate was determined by AA. If the Cu concentration was below 0.05 mg Cu L" 1, a 10- or 15-mL aliquot was evaporated to dryness at 90 C; the residue was dissolved in 1 ml O.lAf HC1 and its Cu concentration was measured by AA. The Cu sorbed was calculated as the difference between that added and that remaining in the equilibrium solution. Copper Desorption Experiment The soils that received 1,200 and 2,000 mg Cu kg 1 during the sorption experiment at ph 5.0 and 8.0 were subsequently used in the desorption study. High Cu addition rates were selected to minimize difficulties and uncertainties in the measurement of low Cu levels in the solution by AA. The centrifuged soils from the sorption experiment (2.0 g + some entrapped solution, which was determined by weighing) were re-suspended in 20 ml 5 mmcacl 2 of ph 5.0 or 8.0, and shaken rigorously for two hrs. Subsequently, the soil suspension was centrifuged, and Cu in the supernatant was determined by AA. The desorption procedure was repeated 10 times. RESULTS AND DISCUSSION Soil Properties That May Affect Copper Reactions Under air-dried conditions, the ph of the soils was rather acidic, ranging from 4.42 in the Rice Hanalei to 5.50 in the Waipio soil. However, after 48 hrs of flooding, soil ph increased significantly by 1.0 to 1.5 units (Table 1). Thus, it is expected that under flooded conditions, the soils would have ph somewhere between 5.0 and 7.0. This information led us to select ph 5.0,6.0,7.0, and 8.0 in constructing the Cu isotherms so that our reacting environment was not much different from that in the field. The rise in ph upon flooding can be partially explained by the reduction of Fe, manganese (Mn), and sulfur (S) minerals, which were probably present in considerable quantities in these soils as reflected by their high oxalate-extractable Fe concentrations (Table 1). A reaction that may be responsible for ph increase by flooding is shown below: FeOOH + H + + e" ==> Fe OH" Clay content was quite high in all of the soils tested, ranging from 460 g-kg 1 in the Maui soil to 890 g-kg- 1 in the Rice Hanalei soil (Table 1). On the other hand, the differences in clay content among soils were only two fold or less. Thus, relative effect of clay on Cu sorption, if any, may not be discernible when comparisons are made among the soils. In contrast, organic C content varied more than 3-fold among the soils, ranging from 21.2 gkg 1 in the Waipio soil to 75.7 g-kg- 1 in the Maui soil. Organic C has been reported to increase Cu sorption, often so strongly that Cu deficiency is common in organic soils (Harter, 1991; Stevenson, 1991).

6 854 HUEETAL. 160 Maui soil.!? 120 <n c o _ Degree 2-Theta FIGURE 1. X-ray diffractogram of the Maui soil (Tropaquept) used in the Cu sorptiondesorption experiment. Copper extracted with 4MHC1 is considered to be a rough estimate of the total Cu in soils, whereas the DTPA-extractable Cu reflects the bioavailable form (Lindsay and Norvell, 1978; Baker and Amacher, 1982). These two forms of Cu and that extracted with 0.1MHC1 were well correlated in our soils (Table 1). For example, the Maui soil which had the lowest total Cu content (24.0 mg-kg" 1 ) also had the lowest DTPA-extractable Cu (5.88 mg-kg 1 ). On the other extreme, the Pearl Harbor-2 soil contained the highest amounts of both total Cu (131.2 mg-kg" 1 ) and DTPA-extractable Cu (34.3 mg-kg 1 ). Copper levels in the 0.1A/HC1 were intermediate between those in 4MHC1 and 5 mmdtpa. A similar finding was reported by Tiller and Merry (1981). Although the total Cu of our soils ( mg-kg 1 ) falls well within the mg Cu kg 1 range reported by Fujimoto and Sherman (1959) for Hawaii soils, it is likely that most Cu in these soils came from anthropogenic activities because the soils' mineralogies are very similar as shown in Figure 1 and Table 2, and as their great group name implies, especially those of the Pearl Harbor series and the Hanalei series. It is reaffirming rather than surprising that these soils have similar mineralogy consisting mainly of primary minerals (mica, feldspars) and some weather able secondary minerals (halloysite, kaolinite). The reason is that these soils are relatively young and have been subjected to similar climatic and moisture conditions for years.

7 REACTIONS OF COPPER SULFATE 855 TABLE 2. Dominant mineralogies of the six wetland-taro soils used in the Cu experiment. Minerals c-spacinp. A Mica or halloysite Halloysite or kaolinite Feldspars , , Kaolinite Copper Sorption as a Function of Time Copper sorption by the soils was rapid. Even at a loading rate as high as 3,000 mg Cu kg* 1 within five minutes of equilibration 76-90% of the added Cu was sorbed (Figure 2). Sorption increased to 81 and 94% after one hr and became relatively stable thereafter (Figure 2). Initial Cu concentrations (and loading rates) also affected Cu sorption: at a lower loading rate of 300 mg Cu kg 1, between 98.0 and 99.9% of the added Cu was sorbed within one hour (Table 3). When the loading rate was 30 mg Cu kg 1 [corresponding to the maximum recommended pesticide application of 1.2 kg of CuSCy5H 2 O ha-cnv 1 of water or 2.7 lb (acrein) 1 ], solution Cu concentrations were all below our A A detection limit (approximately 0.05 mg Cu L 1 ) after one hour of equilibration. The low Cu levels, however, could be estimated by using Table 3 and the assumption that the relationship between added Cu and solution Cu is linear for loading rates <300 mg Cu kg" 1. The predicted values were 0.02,0.03,0.04,0.05,0.06, and 0.06 mg Cu L' 1 for the Maui, Waipio, Pearl Harbor-1, Rice Hanalei, Taro Hanalei, and Pearl Harbor-2 soil, respectively. IfweassumethatO.lOmgCuL" 1 is the minimum concentration of total Cu in solution that is toxic to many living organisms (Baker, 1990), then the following practical implications can be drawn: (i) either the apple snail is very sensitive to Cu or the maximum recommended pesticide application of 1.2 kg of CuSO 4-5H 2 O ha-cm 1 is more effective in some soils than others, or (ii) the recommended waiting time of hrs between CuSO 4-5H 2 O application and crop planting could be shortened to perhaps a day or less (if the water is allowed to drain into down-sloped fields rather than to percolate). Copper Sorption/Desorption Figure 3A shows Cu sorption isotherms at ph 7.0 for the six soils, Figure 3B shows isotherms for the Maui soil at ph 5.0, 6.0, 7.0 and 8.0, and Table 4 lists predicted quantities of Cu sorbed in equilibrium with 0.05, 0.10, or 0.30 mg Cu

8 856 HUEETAL. 1 = Maui 2 = Pearl Harbor-1 4 = Taro Hanalei 5 = Rice Hanalei 3 = Pearl Harbor-2 6 = Waipio Time, minutes FIGURE 2. Effect of equilibration time on percent Cu sorbed by six Hawaii soils (loading rate = 3000 mgcukg-'). TABLE 3. Effect of loading rate on concentration of Cu remaining in solution after 60 minutes of equilibration. Cu Initial Cu* Cu remaining in solution Loading cone. rate (added) R. Hanalei 1 T. Hanalei Maui PH-1 PH-2 Waipio mg kg' 1 0* < mg Cu L" Soil-to-solution ratio was 1:10; equilibrating solution was 5 mmkjs0 4. 'Taken from the Cu sorption isotherm experiment. 'Rice Hanalei, Taro Hanalei, Maui, Pearl Harbor-1, Pearl Harbor-2, and Waipio soils.

9 REACTIONS OF COPPER SULFATE 857 o D A Rle«Hanakl Taro HanaUl P»arl Habror-t Pvarl Hatbor-1 Maui Walplo o 0) " " Copper in solution, mg Cu/L FIGURE 3. Copper sorption as influenced by soil type (A) and soil ph (B). L" 1 in solution. These soluble Cu concentrations were used as reference points because they represent levels that may be toxic to living organisms (Baker, 1990). As shown in Table 4 and Figure 3A, the Maui soil generally sorbed the most Cu, followed by the Waipio, Pearl Harbor-1, Pearl Harbor-2, Taro Hanalei, and Rice Hanalei soil in this order. For example, at ph 6.0 and 0.10 mg Cu I/ 1 in solution, the Maui soil must have sorbed 500 mg Cu kg 1, the Waipio soil 350, the Pearl Harbor-1400, the Pearl Harbor-2 310, the Taro Hanalei 260, and the Rice Hanalei

10 858 HUE ET AL. TABLE 4. Quantities of Cu sorbed by the six wetland-taro soils of Hawaii. Soil Equilibrium ph 0.05 Sorbed Cu* * < mg Cu kg "' > Maui Taro Hanalei Rice Hanalei Pearl Harbor-1 Pearl Harbor-2 Waipio Quantities of Cu sorbed in order to maintain 0.05,0.10, or 0.30 mg Cu I/ 1 in solution. Concentration of Cu in the equilibrium solution in mg Cu L" mg Cu kg 1. If we assume that on the average, cultural practices for wetland taro would add 2.5 mg Cu kg' (5 lb Cu acre') annually to the soil (four applications of CuSO 4-5H 2 O at 1 Jb Cu acre 1 application 1 to control snails and 1 lb Cu acre 1 from fertilizer impurities), then the apple snail control with CuSCy5H 2 O can be tolerated for at least 84 years in the Rice Hanalei soil and as many as 200 years in the Maui soil before Cu biotoxicity becomes a concern.

11 REACTIONS OF COPPER SULFATE Equilibrium ph Maul Waipio Pearl Harbor-1 Pearl Harbor-2 Taro Hanalei Rice Hanalei FIGURE 4. Quantities of Cu sorbed by the six wetland-taro soils of Hawaii when the concentration of total Cu in solution was 0.10 mg Cu L" 1. Increasing ph generally increased Cu sorption (Figures 3B and 4). The sharpest increase in sorption occurred between ph 6.0 and 7.0, perhaps because most organic molecules and oxidic soil minerals change their surface charge from positive or neutral to negative at this ph range; and the retention of Cu, mostly as Cu 2+, is much stronger by negatively charged surfaces than positively charged ones. A practical implication of this effect is that Cu concentration in soil solution, and hence biotoxicity of added Cu, can be regulated by changing soil ph: raising soil ph to 7.0 or slightly above increases Cu sorption and thus reduces its potential toxicity to crops or non-targeted aquatic organisms. If toxicity should occur, flooding the field for hrs before planting crops could alleviate this problem. Copper sorption was not correlated with soil properties, such as clay content, oxalate-extractable Fe, or organic C, perhaps because the number of soils tested was small (six soils) and they were similar in many properties. Nevertheless, there seems to be a relationship albeit weak between sorbed Cu and the indigenous Cu as extracted by 4MHC1 or 5 mmdtpa coupled with the organic matter content of the soil Tables 1 and 4). The amounts of Cu sorbed tend to be low if the indigenous Cu is high and organic C is low. O j I

12 860 HUEETAL. ph 5.0 Cumulative leaching volume, ml g' 1 ph * <0 60 SO Cumulative leaching volume, ml g' 1 FIGURE 5. Cumulative Cu desorption by CaCl 2 solution as a percentage of Cu sorbed by six wetland-taro soils of Hawaii. Copper desorption is illustrated in Figure 5. Only a small portion of sorbed Cu was desorbed, even during 10 consecutive extractions with 5 mm CaCl 2 at a 1:10 soil-to-solution ratio (weight-to-weight basis). Only 5% and 1.2% of sorbed Cu in the Maui soil were desorbed at ph 5.0 and 8.0, respectively. Desorption was relatively higher (30% at ph 5.0 and 10% at ph 8.0 of the sorbed Cu) in the weak

13 REACTIONS OF COPPER SULFATE 861 Cu-sorbing Rice Hanalei soil. There appears to be an inverse relationship between Cu sorption and desorption by the soils tested: the more Cu a soil can sorb, the tighter it holds Cu and the less Cu it releases. SUMMARY A Cu sorption/desorption study was conducted to estimate the environmental impact of CuSCy5H 2 O when used as a snail controlling pesticide. Six soils representing major wetland-taro growing areas in Hawaii were used. To reach a soil-solution Cu concentration of 0.10 mg Cu L" 1, a level deemed harmful to some Cu-sensitive organisms, an addition of 500 mg Cu kg" 1 is required for the Maui soil, 400 for the Pearl Harbor-1 soil, 3 50 for the Waipio soil, 310 for the Pearl Harbor-2 soil, 260 for the Taro Hanalei soil, and 210 for the Rice Hanalei soil. If we assume an average annual Cu input of 2.5 mgkg -1 (5 lbacre 1 ) to Hawaii soils planted to wetland-taro, then it would take from 84 to 200 years under current practice to build the soil Cu up to a level of concern. The Hanalei soils were most vulnerable, the Maui soil least. The Waipio soil on the Big Island and the Pearl Harbor soils from Kahaluu, Oahu, were intermediate. Increasing soil ph to approximately 7.0 increased Cu sorption, thereby reducing potential Cu biotoxicity. Thus, the recommended practice of flooding the soil for at least 48 hrs before planting taro should be closely followed. It is difficult to predict Cu sorption from common soil properties, such as clay content or soil-test Cu. Nevertheless, it appears that more Cu is sorbed (less Cu remained in solution) if the soil contains high organic C and low indigenous Cu. REFERENCES Baker, D.E Copper, pp In: B.J. Alloway (ed.), Heavy Metals in Soils. Blackie and Son, Glasgow, UK. Baker, D.E. and M.C. Amacher Nickel, copper, zinc, and cadmium, pp In: A.L. Page, R.H. Miller, and D.R. Keeney (eds.), Methods of Soil Analysis. Part 2. American Society of Agronomy, Inc., Madison, WI. Basta, N.T. and M.A. Tabatabai Effect of cropping systems on adsorption of metals by soils. II. Effect of ph. Soil Sci. 153: Birge, W.J. and J.A. Black Effects of copper on embryonic and juvenile stages of aquatic animals, pp In: J.O. Nriagu (ed.), Copper in the Environment. Part II: Health Effects. John Wiley & Sons, New York, NY. Cheng, T.C Use of copper as a molluscicide, pp In: J.O. Nriagu (ed.), Copper in the Environment, Part II: Health Effects. John Wiley & Sons, New York, NY.

14 862 HUEETAL. Fujimoto, G. and G.D. Sherman The copper content of typical soils and plants of the Hawaiian Islands, pp In: Technical Progress Report 121. Hawaii Agricultural Experiment Station, University of Hawaii, Honolulu, HI. Gee, G.W. and J.W. Bauder Particle-size analysis, pp In: A. Klute (ed.), Methods of Soil Analysis. Part 1. American Society of Agronomy, Inc., Madison, WI. Harter, R.D Micronutrient adsorption-desorption reactions in soils, pp In: J.J. Mortvedt, F.R. Cox, L.M. Shuman, and R.M. Welch (eds.), Micro-Nutrients in Agriculture. Soil Science Society of America, Inc., Madison, WI. Lindsay, W.L. and W.A. Norvell Development of a DTPA soil test for zinc, iron, manganese, and copper. Soil Sci. Soc. Am. J. 42: McBride, M.B Forms and distribution of copper in solid and solution phases of soil, pp In: J.F. Loneragan, A.D. Robson, and R.D. Graham (eds.), Copper in Soils and Plants. Academic Press, New York, NY. Msaky, J.J. and R. Calvet Adsorption behavior of copper and zinc in soils: Influence of ph on adsorption characteristics. Soil Sci. 150: Nelson, D.W. and L.E. Sommers Total carbon, organic carbon, and organic matter, pp In: A.L. Page, R.H. Miller, and D.R. Keeney (eds.), Methods of Soil Analysis. Part 2. American Society of Agronomy, Inc., Madison, WI. Stevenson, F.J Organic matter-micronutrient reactions in soil, pp In: J.J. Mortvedt, F.R. Cox, L.M. Shuman, and R.M. Welch (eds.), Micronutrients in Agriculture. Soil Science Society of America, Inc., Madison, WI. Tiller, K.G. and R.H. Merry Copper pollution of agricultural soils, pp In: J.F. Loneragan, A.D. Robson, and R.D. Graham (eds.), Copper in Soils and Plants. Academic Press, New York, NY.