Autoclave decomposition method for metals in soils and sediments

Transcription

1 Environ Monit Assess (2012) 184: DOI /s Autoclave decomposition method for metals in soils and sediments M. Navarrete-López M. P. Jonathan P. F. Rodríguez-Espinosa J. A. Salgado-Galeana Received: 20 November 2010 / Accepted: 26 April 2011 / Published online: 17 May 2011 Springer Science+Business Media B.V Abstract Leaching of partially leached metals (Fe, Mn, Cd, Co, Cu, Ni, Pb, and Zn) was done using autoclave technique which was modified based on EPA 3051A digestion technique. The autoclave method was developed as an alternative to the regular digestion procedure passed the safety norms for partial extraction of metals in polytetrafluoroethylene (PFA vessel) with a low constant temperature (119.5 ± 1.5 C) and the recovery of elements were also precise. The autoclave method was also validated using two Standard Reference Materials (SRMs: Loam Soil B and Loam Soil D) and the recoveries were M. Navarrete-López J. A. Salgado-Galeana Escuela Nacional de Ciencias Biológicas, Laboratorio Central de Instrumentación, Instituto Politécnico Nacional (IPN), Prol. de Manuel M Carpio y Plan de Ayala, Col. Sto. Tomas, Del. Miguel Hidalgo, C.P.11340, Mexico D.F., México M. Navarrete-López mnavarretel@hotmail.com M. P. Jonathan (B) P. F. Rodríguez-Espinosa Centro Interdisciplinario de Investigaciones y Estudios sobre Medio Ambiente y Desarrollo (CIIEMAD), Instituto Politécnico Nacional (IPN), Calle 30 de Junio de 1520, Barrio la Laguna Ticomán, Del. Gustavo A. Madero, C.P.07340, Mexico D.F., México mpjonathan7@yahoo.com equally superior to the traditionally established digestion methods. Application of the autoclave was samples from different natural environments (beach, mangrove, river, and city soil) to reproduce the recovery of elements during subsequent analysis. Keywords Leaching Validation Autoclave Soils Sediments Introduction Soil/sediment samples are often extensively used for estimation of metal concentration levels in industrial/agricultural outputs, geochemical exploration, and to identify environmental contamination. The sediments normally consist of different mixtures of organic material and minerals. The sediments are often transported through various ways and are deposited in lakes, rivers, dams, and in marine regions in a stratified manner. The humic substances in the sediment (organic substance) help to accumulate the trace metals in the surface layers via ionic inter-exchange and chelating process (Van Loon and Barefoot 1989). During the depositional process in aquatic environment, oxides and hydroxides of Fe, Mn, and Al often trap trace metals, which are subsequently accumulated as hydroxides.

2 2286 Environ Monit Assess (2012) 184: Table 1 Comparison of different parameters of EPA leach methods with that of the present autoclave leach method Method Method SW-846 New method (present study) Method status Revised 2/1996 Revised 0/1994 Revised 1/2007 Nil Method EPA 3050B EPA 3051 EPA 3051A Autoclave assisted Matrix Sediments/soils (sludge) Sediments/soils Sediments/soils Sediments/soils (sludge and oils) [10, 11] (sludges and oils) [10, 11] (a) (b) Digestion reagents HNO3 + H2O2 HNO3 + HCl HNO3 HNO3 HNO3 + HCl HNO3 HNO3 + HCl Volume of reagents Series of additions 10 ml 10 ml ml 10 ml ml Sample mass (g) 1 2 g g g g Digestion vessels Plastic or glass open method PFA or TFM closed PFA or TFM closed (V 45 ml) PFA or TFM closed (120 ml) (250 ml) (120 ml) Withstanding inner P 1atm 7.5± 0.7 atm 30 atm 7.5 ± 0.7 atm pressure in vessel (110 ± 10 psi) (435 psi) (110 ± 10 psi) Outer working pressure P 1atm(14.7psi) P 1 atm (14.7 psi) P 1 atm (14.7 psi) P 2.4 atm (34.8 psi) Heating source Hot plate, block digester, Microwave unit Microwave unit 600 1,200 W Autoclave unit 1,050 or 1,600 W microwave etc ,200 W with with rotating turn table with thermostat rotating turn table Pre-digestion** Nil In open vessel In open vessel In open vessel Heating digestion Series of 95 C ± 2.5 refluxes Heat for 175 ± 5 C in Heat for 175 ± 5 C in 5.5 min Heat at C ± 1.5 Cand program 2 8 h or more 5.5 min and hold it and hold it for 4.5 min more hold it for 40 min for 4.5 min more % of maximum 10% 1% 1% accepted lost sample Instruments used GFAA, ICP-AES, FLAA, GFAA, FLAA, GFAA, ICP-AES, FLAA, GFAA, ICP-MS, FLAA, for measurement ICP-MS, FLAA ICP-AES ICP-MS, ICP-AES ICP-MS, FLAA ICP-AES ICP-OES, FLAA ICP-OES Other application (EPA Method 3050B 1996) For samples which are highly reactive (5 10% of organic material) (EPA Method 3051A 2007; EPA Method )

3 Environ Monit Assess (2012) 184: The estimation of metal concentrations in sediments often depends on the type of digestion procedures used for extracting the metals (Binstock et al. 1991; Medved et al. 1998; Pérez Cid et al. 2002; Wanekaya et al. 2002; Arslan and Tyson 2008). Normally, for total digestion of metals (e.g., Pb, Cu, Zn, Mn, Cd, Co, Ni, etc.,) HF is often used to extract the complete matrix. During the past two decades the rapid decomposition of sediments was done using closed vessel digestion technique. The strong extracts of HF often contain concentrated form of HNO 3 and HCl to attack the different bonds of sediment matrix in the basic structure (Kingston and Jassie 1988; Van Loon and Barefoot 1989; Medved et al. 1998; Potts 1992). These types of acid mixture often help to digest the absorbed and interexchangeable metals through ionic reaction and co-precipitation with hydroxides. However, the mixture of HNO 3 and HCl acids do not attack the basic structure of the matrix in the sediments, where major components such as silicates, quartz, titanium dioxides, aluminum, and other oxides are not dissolved. In majority of the sediments, the immobile nature of the matrix often carries the contaminants. The extraction procedures with soft acids are often temperature dependent to remove a particular fraction of metals only (Ya-Guerasimov et al. 1977). The EPA has efficiently employed different decomposition methods for digestion of sediments. The digestion procedures mentioned in EPA SW- 846 contain 3050B, 3051, and 3051A decomposition methods are applied in different conditions for extraction of metals (Table 1). The EPA 3050 method extracts metals in low condition (open digestion) with HNO 3 H 2 O 2 (3050A) acids and with HNO 3 HCl (3050B) combinations. In the EPA 3051 method, a closed microwave technique was used, where HNO 3 is used exclusively to minimize the interference from HCl during analytical measurements. The EPA 3051A method (developed in 1998 and revised recently in 2007) is a closed microwave digestion technique, where HNO 3 HCl and HNO 3 mixtures are used separately for extraction of metals. In all the techniques, the extracts were measured using ICP- AES, FLAA, and GFAA and ICP-MS, respectively. The difference in the above three methods is the change in use of energy conditions and change in acids and pressure, which often affects the total costs of the methodology (Zhuchenko et al. 2008). This article describes the use of autoclave decomposition method, where it is used with better safety, temperature control, and the number of metals in a single leach (Fe, Mn, Cd, Co, Cu, Ni, Pb, and Zn) with optimum leach recoveries. The article also reports in the concluding part the advantages of the autoclave method in comparison to traditional methods of EPA 3050, 3051, and 3051A (Table 1). Experimental section Samples used The present study of autoclave leach method was performed using the All American autoclave unit model (75X). The reagent blanks (concentrated acids) were acquired from J.T.Baker (ACS Instra Grade) and Millipore reagent water was used throughout the analysis (Annual Book of ASTM Standards 1985). The SRMs used for calibration (CCV-1 and ICP A) in this study was purchased from High Purity Standards (HPS), Charleston, USA. The SRMs (SRM Loam Soil B Cat.No.CRM- LO-B, Lot.No ; SRM Loam Soil D Cat.No.CRM-LO-D, Lot.No ) used for validation of the present autoclave method were purchased from HPS. In the present experimental work, the soils/sediments were selected from four different regions to know the recovery ranges and to prove that acid leacheable autoclave method can be adapted to all natural regions. A set of 20 samples (five from each area) from (1) beach, (2) mangroves, (3) river area, and (4) city soil with different textural characters were used to validate the recovery of elements. The main objective on the use of diverse samples was to know the use of autoclave method in all regions. The validation of the autoclave leach method in the present study was done by using two different solid SRMs as per the specified methods (Table 1) (USEPA 1992). The two different SRMs (loam B

4 2288 Environ Monit Assess (2012) 184: and loam D) were used to validate the autoclave leach method for various other parameters like extraction time, vessel pressure (for digestion), loss of sample (during extraction), and for overall performance on extraction of elements. Digestion vessel used The digestion vessel used in the present experiment is a commercially sold closed vessel, which is made of a resistant material [poly (tetrafluoroethylene)] PFA sold by SAVILLEX and it was purchased from Analytical Control, México, D.F (USEPA 1992; Taylor and John 1987; Analytical Control 2009). The internal volume of the vessel is 120 ml, which is capable of withstanding a net pressure of at least 5.1 atm (75 psi) for nitric acid and 6.8 atm (100 psi) for HNO 3 + HCl acid mixture, where the pressure is controlled by a pressure relief valve. To minimize the potential contamination, all PFA vessels and lab wares were cleaned using either hot or normal acids prior to each leach analysis. Instruments used The analysis in the present study was done using ICP-OES (Varian 720ES) for Fe, Mn, Cd, Co, Cr, Ni, Pb, and Zn and the whole procedure was done in Escuela Nacional de Ciencias Biológicas, Laboratorio Central de Instrumentación, Instituto Politécnico Nacional in Mexico City, which is authorized by EMA (Mexican Organization for Authorization; no. AG /06). The internal chemical solutions and SRMs for analysis were prepared under the same working conditions. Results and discussion The results of the present study are described briefly below to understand the advantages of the autoclave digestion procedure and maximum number of elements that can be analyzed in precision (Table 1). The whole methodology was extracted and analyzed completely five times to have accurate results in extraction and validation of this new autoclave method. Optimization of digestion time The time required for digestion of soils/sediments in the present study was optimized using the closed digestion vessel, where the valve withstood a pressure of 7.5 atm (110 psi). The sample used in the present study was Buffalo River Sediment (SRM 8704) and the comparison of the two different leach methods is presented in Fig. 1. The leaching time for the recovery of elements from soils/sediments was varied from 20, 40, 60, and 80 min, respectively. The maximum leaching during different tests was achieved during 40 min of leaching for most of the elements using both the acid mixtures. The leaching of elements was better when using HNO 3 + HCl mixture than HNO 3 acid alone (Fig. 1). The instrumental interferences were higher during the first 20 min leaching than in 40 min. In both the digestion mixtures, the leaching had stabilized during the first 40 min, and as the leaching time was increased the interferences during reading (in ICP-OES) was also high. Moreover, during the 40 min digestion time, 80% to 100% of the metals were extracted with respect to the concentration of elements leached by EPA 3050B method with a low standard deviation and good instrumental precision. Temperature In the autoclave digestion system a thermostat was used to control the temperature, which plays a major role in the leaching of samples. In the present experiment, the temperature was controlled at ± 1.5 C, compared to the open method (EPA 3050), where it was ±2.5 Cand ±5 C in microwave digestion (EPA 3051 and 3051A) (Ya-Guerasimov et al. 1977; EPA Method 3051A 2007; EPA Method ). The temperature control in digestion of samples is an important parameter, which is used to validate the autoclave method and is also supported by the precision and duplication during the extraction of metals (EPA Method 3051A 2007; Dirk et al. 1998, 1999). Safety The present study using autoclave method involves heating the reagents in closed PFA vessels

5 Environ Monit Assess (2012) 184: Fig. 1 Recovery of elements (in %) using HNO 3 and HNO 3 + HCl acid digestion on SRM (Loam Soil D) and SRM (Loam Soil B) and the safety of the working condition should be taken into account. Two types of acid mixture was used in the experiment, where HNO 3 was used first and secondly the HNO 3 + HCl mixture which causes additional pressure generating nearly thrice to that of HNO 3 acid was used (EPA Method 3051A 2007; Dirk et al. 1998). In this experimental study, the pressure of the digestion vessel was varied manually to three different levels, where it withstood pressures of 5.1 atm (75 psi), 6.8 atm (100 psi), and 8.5 atm (125 psi) compared to the traditional methods like EPA 3051 [7.5 atm (110 psi)] and 3051A [30 atm (435 psi)] (Table 1). The main criteria for accepting a digestion vessel for regular use is the percent loss of sample (less than 1%), which conform to EPA 3051A method, In this study, the first digestion mixture (Table 1) was of 0.5 g of solid soil/sediment sample mixed with 10 ml of HNO 3 acid mixture and heated to ± 1.5 C in the closed vessel, which generated less than 5.1 atm pressure. During this digestion period, the sample loss was less than 1% during the 40-min leaching time as the minimum pressure to open the valve was also above 5.1 atm (75 psi). The same digestion procedure was also applied using different digestion vessels of 6.8 atm (100 psi) and 8.5 atm (125 psi), respectively (Fig. 2). The second acid mixture (Table 1), a 0.5 g of solid soil/sediment sample was mixed with 9:3 acid mixture (9 ml HNO 3 + 3mlHCl) and digested at ± 1.5 C in a closed vessel generating a pressure of more than 5.1 atm (75 psi) during the same 40-min leaching period (Fig. 2). The sample loss during this digestion period at 5.1 atm (75 psi) was 2.6%. However, when the vessel pressure was increased to 6.8 atm (100 psi) and to 8.5 atm (125 psi) the loss of sample came down below to 1% (Fig. 2). The above process indicates that it is better to use a vessel which withholds a pressure of 5.1, 6.8, and 8.5 atm when using HNO 3 acid alone for autoclave digestion. Likewise, when using 9:3 acid mixture (HNO 3 + HCl),anautoclave digestion vessel withholding 6.8 and 8.5 atm is recommended for better precision. In both the cases, the loss of sample was less than 1% even though the pressure increased with change of acid mixtures. The samples used (SRM Loam Soil B; SRM Loam Soil D; beach; mangroves; river area; city soil) in the present study were analyzed using both the acid mixtures separately to demonstrate the

6 2290 Environ Monit Assess (2012) 184: ml HNO3: 3 ml HCl 40 min. Subsequently, the vessel was cooled in room temperature and weighed again to identify the loss of sample weight (EPA Method 3051A 2007). Loss of sample (%) ml HNO3 Acceptable Limit Pressure (psi) Fig. 2 Typical pressure and percentage loss of sample profiles for the two different acid digestion techniques adopted in the present study (using 3051A) application of autoclave method. The PFA vessel was pre-weighed along with the sample, sealed tightly, and it was heated to ± 1.5 Cfor Predigestion The predigestion of solid soil/sediment sample is necessary if the sample loss is more than 1%, which is due to volatile and easily oxidized compounds present in the solid sample (EPA Method 3051A 2007). The predigestion in open vessel is done by heating it on a sand (or) water bath for 40 to 60 C or 0.2 to 2 hours and it is sealed for further digestion. The above process help the gases generated to escape and subsequently the autoclave vessel can be heated to 119. ± 1.5 Cfor 40 min. Calculation of uncertainty in soil/sediment analysis The sources for calculating the uncertainty in the experimental section or estimation is done based on the following scientific fundamentals: temperature, pressure, and time. u(cmi(mg/g)) CMi(mg/g) { = } 2 u( CE0Mi) + CE0Mi { } u(v0) 2 { } u(m) 2 { } u( fd) 2 { } u( fc) (1) V0 m fd fc (a) (b) (c) (d) (e) (f ) Where a f is related to the uncertainty calculations (Taylor and John 1987; Zhuchenko et al. 2008; CENAM EMA 2004). The Eq. 1 is expressed as: (b) for calculating the concentration (milligrams per liter) in the extract during measurement by the instrument; (c) is the extracted volume, which depends on three variables, where the first one is the replicates of volume by the analyst, secondly the variation of temperature during measurement of volume/calibration, and thirdly, the tolerance volume limit of the material. Likewise, (d) is the mass of the soil/sediment and (e) is the dilution factor ( f d = V f /V i ), which depends on the diluted volume( as mentioned ) in (c). The factor for correction f c = ŝ ŝ,(f) depends on the dilution of SRM ) for obtaining the concentration of spike (ŜMi and the reading of the ( ) spike in the instrument ŜMi. Finally, (a) is the concentration of the element for mass per gram C Mi (milligrams per gram) for soil/sediment and it is closely related to the uncertainty standard U[CMi (milligrams per gram)] (CENAM EMA 2004; Eurachem 2000; EPA Method 3051A 2007).

7 Environ Monit Assess (2012) 184: Table 2 Comparison of elemental recoveries through autoclave digestion method from SRM (Loam Soil B) using HNO 3 and HNO 3 + HCl mixtures Methods Reported EPA Autoclave digestion Autoclave digestion 9 ml Total digestion 3050A Loam Soil B 10 ml HNO 3 mixture HNO ml HCl mixture (reported) (reported) (present study) (present study) (a) (b) (c) Elements % Rec. % Rec. Fe 22.3 ± 0.8 mg/g 23.7 ± 1.7 mg/g ± 1.85 mg/g ± 0.4 mg/g Mn 1.47 ± 0.16 mg/g 1.23 ± 0.09 mg/g ± 0.09 mg/g ± 0.04 mg/g Cd 83 ± 5 μg/g 79 ± 5 μg/g ± 5 μg/g ± 7 μg/g Co 59 ± 3 μg/g 52.6 ± 4.9 μg/g ± 4 μg/g ± 3 μg/g Cu 52 ± 4 μg/g 53.6 ± 4 μg/g ± 4 μg/g ± 3 μg/g Ni 54 ± 4 μg/g 55.7 ± 9 μg/g ± 8 μg/g ± 4 μg/g Pb 95 ± 3 μg/g 95 ± 11 μg/g ± 12 μg/g ± 6 μg/g Zn 206 ± 10 μg/g ± 23 μg/g ± 22 μg/g ± 20 μg/g Application and validation to real samples The use of autoclave digestion method is reported as another option for the partial extraction of metals was applied to two soil samples (SRM Loam Soil B and SRM Loam Soil D). This was applied in order to evaluate the application of the autoclave method and its recovery percentage during the digestion of samples. The results generated from the autoclave digestion for Fe, Mn, Cd, Co, Ni, Pb, and Zn from both the SRMs are presented in Tables 2 and 3. The recovery of elements with respect to EPA 3050A for Loam Soil B using HNO 3 digestion were 84% to 106% (Table 3) and using 77% to 115% (Table 3) usinghno 3 + HCl acid mixture digestion (Table 2). The recoveries of elements were calculated based on the reported values by EPA 3050A and 3050B. Likewise, the recovery of elements using HNO 3 digestion for Loam Soil D was 94% to 119% (Table 3) and 84.1% to 119% (Table 3) usinghno 3 + HCl acid mixtures. The overall performance of the recovery of elements using both the digestion mixtures indicates superior precision and recovery of elements. The autoclave digestion method was applied to naturally available sediments from beach, mangrove, river, and city soils (Table 4) to prove and replicate the elements during each analysis. The samples were completely analyzed five instances Table 3 Comparison of elemental recoveries from SRM (Loam Soil D) using HNO 3 and HNO 3 + HCl digestion mixtures Methods EPA 3050A Autoclave digestion EPA 3050B Autoclave digestion Total digestion HNO 3 + H 2 O 2 10 ml HNO 3 mixture HNO 3 + HCl 9 ml HNO ml HCl (reported) mixture (reported) (present study) mixture (reported) mixture (present study) (a) (b) (c) Elements % Rec. % Rec. Fe 38.1 mg/g ± 2 mg/g mg/g ± 3.4 mg/g ± 0.57 mg/g Mn 3.25 mg/g 3.84 ± 0.25 mg/g mg/g 3.8 ± 0.24 mg/g ± 0.07 mg/g Cd a Co 28.6 μg/g ± 3 μg/g μg/g 27.6 ± 3.4 μg/g ± 1.8 μg/g Cu 97.5 μg/g 95 ± 5.4 μg/g μg/g 98.5 ± 5.7 μg/g ± 3 μg/g Ni 38 μg/g 37.8 ± 7.7 μg/g μg/g 39.6 ± 8.3 μg/g ± 3.7 μg/g Pb 119 μg/g 112 ± 11 μg/g μg/g 111 ± 13.4 μg/g ± 6 μg/g Zn 76.4 μg/g ± 9.9 μg/g μg/g 85.6 ± 10 μg/g ± 7 μg/g a As there is no reported values for SRM Loam Soil D we have not included our values

8 2292 Environ Monit Assess (2012) 184: Table 4 Application of autoclave method (HNO 3 mixture and HNO 3 + HCl mixture) in samples from different environments Methods Autoclave digestion present study Autoclave digestion present study 10 ml HNO 3 mixture 9 ml HNO 3 + 3mlHClmixture (a) (b) Elements/ Beach Mangrove River City soil Beach Mangrove River City soil places Fe (mg/g) 1,519 ± ,995 ± ,501 ± ,992 ± ,528 ± ,005 ± ,508 ± ,081 ± 16.1 Mn (mg/g) 94 ± ± ± ± ± ± ± ± 8.52 Cd (μg/g) 0.90 ± ± ± ± ± ± ± ± 0.12 Co (μg/g) 3.05 ± ± ± ± ± ± ± ± 0.15 Cu (μg/g) ± ± ± ± ± ± ± ± 1.45 Ni (μg/g) 8.42 ± ± ± ± ± ± ± ± 1.90 Pb (μg/g) ± ± ± ± ± ± ± ± 4.58 Zn (μg/g) ± ± ± ± ± ± ± ± 5.28 The samples (five each from different environments) were analyzed five times completely from the beginning to know the reproducibility on the recovery of elements from the starting till the end as a new batch of sample. The results indicate good reproducing capability without much deviation in the analytical readings which substantiates the claim that the digestion method can be used in natural samples for rapid analysis. Conclusion The results from the experimental work in developing the new autoclave method indicate the following advantages: 1. The safety aspects of the digestion technique passed the specified norms for temperature, pressure, and time for partial digestion of soil/sediment samples. 2. The optimum time for digestion was generated to obtain the maximum recovery of elements. 3. The temperature control in the study was established at ± 1.5 C, which is much lower than other traditional methods and it also attains maximum acceptable recovery of elements with a low risk of vessel explosion. 4. The uncertainty calculation in the soil/ sediment analysis is formulated and calculated to have precision in the analysis. 5. The leach results of Loam Soil B and Loam Soil D indicate good recovery, equally to the already published values indicating improvement in recovery of elements. 6. The application of autoclave method to natural soil and sediments also indicate good recovery of elements and reproducing capability. 7. The autoclave method emphasizes the use of commercially available equipment at low cost and high technology (USEPA 1992; Taylor and John 1987). The data presented in this article provides additional leach methods with various advantages and more elements in a single digestion technique. Acknowledgements We thank Varian, Mexico, for their support and for providing ICP-OES instrument. We also wish to express our gratitude to EDD, EDI, and COFAA- IPN. MPJ wishes to thank SNI-CONACyT, Mexico. MPJ & PFRE thanks SIP-IPN projects awarded to them (2009 & 2010). This article is the 53rd partial contribution from Earth System Science Group (ESSG), Chennai, India. References Analytical Control (2009). S.A. de C.V., Renan, , Col. Anzures, Mexico, D.F., C.P Annual Book of ASTM Standards (1985). Standards specif ication for reagent water (pp. D ). Philadelphia, PA, ASTM. Arslan, Z., & Tyson, J. F. (2008). Determination of trace elements in siliceous samples by ICP-MS after precipitation of silicon as sodium fluorosilicate. Microchim Acta, 160,

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