доц. д-р Тони Венелинов, Хидротехнически Факултет, УАСГ

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1 доц. д-р Тони Венелинов, Хидротехнически Факултет, УАСГ ИЗПОЛЗВАНЕ НА РАЗРЕДЕНА КИСЕЛИННА СМЕС ЗА МИКРОВЪЛНОВО РАЗЛАГАНЕ ПРИ ИЗМЕРВАНЕТО НА ОБЩОТО СЪДЪРЖАНИЕ НА Cr, Cu, Mn И Zn ПРИ ОХАРАКТЕРИЗИРАНЕТО НА СЕРТИФИЦИРАН РЕФЕРЕНТЕН МАТЕРИАЛ ЗА УТАЙКА USE OF DILUTED ACID MIXTURE FOR MICROWAVE-ASSISTED DIGES- TION PRIOR TO DETERMINATION OF THE TOTAL CONTENT OF Cr, Cu, Mn, AND Zn WITHIN THE CHARACTERIZATION OF A CERTIFIED REF- ERENCE MATERIAL FOR SEWAGE SLUDGE T. Venelinov, assoc. prof. Ph.D., tvenelinov_fhe@uaceg.bg Faculty of Hydraulic Engineering, UACEG Резюме: Представен е метод за определяне на хром, мед, манган и цинк в утайки, при който се използва разредена киселинна смес, съдържаща 20% азотна киселина и 5% флуороводородна киселина, в комбинация с микровълново нагряване (до 240 C) при високо налягане (до 150 bar) според дефинирана температурна програма. Представени са стъпките за избор на киселинната смес при разработването на метода за разлагане. Концентрацията на микроелементите в разложените проби е определена чрез индуктивносвързана плазма оптично емисионна спектрометрия. Методът е валидиран и са определени неговите характеристики. Той е успешно приложен при анализ на сертифицирани сравнителни материали за седименти и утайки в рамките на измерванията за хомогенност, краткосрочна и дългосрочна стабилност и за приписване на сертифицирани стойности, както и в междулабораторно сравнение при изпитване на пригодност. Ключови думи: Утайки, микроелементи, СРМ INTRODUCTION Soils, sediments and sewage sludges are important environmental materials. The safe use of sewage sludge in agriculture is regulated by the council directive 86/278/EEC [1]. Limit values for concentrations of Cd, Cu, Hg, Ni, Pb and Zn in soil and sewage sludge are established in such a way as to prevent harmful effects on soil, vegetables, animals, and man. The trace element content in soils is important because of their migration into plants by the root system. Trace element accumulation in plants is directly linked to their bioavailability to humans and animals when entering the food-chains. The sediments are in a constant exchange with the water column above and actively contribute to the removal or uptake of pollutants from the water, providing a historical record of water quality or pollution. Therefore accurate data from the analysis of such materials, produced by numerous laboratories worldwide on daily basis, are required for a number of purposes, especially for monitoring activities to evaluate their potential health and environmental risk. Certified Reference Materials (CRMs) are essential in providing comparability of data obtained in different parts of Europe and world since the quality of analytical data can have financial and ecological impacts. The common approach for sewage 17

2 sludge sample determination includes the determination of so-called operationally defined parameters, which are strongly influenced by the extraction protocol BCR sequential extraction procedure [2-4] and aqua regia extraction [5,6], both often referred to as pseudo-total content, which is considered as an adequate estimation of the bioavailable trace element fraction. The total element content determination will use digestion methods that employ the use of concentrated acids [7-10], and in very limited cases diluted acid mixtures for heavy metal removal [11-14], generating low blank levels and low relative standard deviations. Compared to concentrated acid mixtures, diluted acids generate fewer residues and do not require high dilution factors prior to analyte measurements, making the analysis easier, faster and cheaper, according to the green chemistry trends [15]. MATERIALS AND METHODS Sample Preparation A 10 ml mixture of 20 % v/v of nitric acid and 5 % v/v hydrofluoric acid is added to about 0,10 g sub-sample of the sewage sludge certified reference material BCR 144R (IRMM) with certified values 104,3 ± 2,4 mg/kg for total Cr, 308 ±7 mg/kg for total Cu, 207,9 ± 2,5 mg/kg for total Mn and 932 ± 23 mg/kg for total Zn contents using analytical microbalance (Mettler Toledo AX504 with accuracy of 0,0001g). The vessels with the samples are then placed into a microwave reactor of the microwave UltraCLAVE (Milestone, US). The reaction vessels were pressurized to 100 bar with nitrogen gas. The maximum applied power for all steps is 1000 W, and the set maximum temperatures are 120 C for 45 min. at 100 bar pressure, and 240 C for 45 min. at 150 bar pressure. ICP-OES Measurement Measurements for Cr, Cu, Mn, and Zn in sewage sludge materials were performed on ICP-OES IRIS Intrepid II XDL (Thermo). Measurement and control lines were used for the ICP-OES determination of Cr (267,716 and 205,552), Cu (324,754), Mn (257,610 and 260,569) and Zn (202,54 and 206,200). Optimal instrumental parameters are presented in table 1. Table 1. Analysis preferences and source settings for the ICP-OES measurements of trace elements in sewage sludge materials Method validation According to ISO [16], validation is the confirmation by examination and the provision of objective evidence that the particular requirements for a specific intended use are fulfilled. It requires that the laboratory validates non-standard methods, laboratory-developed methods, standard methods used outside their intended scope, and modified of standard methods to confirm that they are fit for their intended use. Extensive validation is necessary to establish that the method of choice is under statistical control each time 18

3 the method is used, so it fully meets the needs of the intended application. Targeted method performance criteria for the determination of the trace elements in sewage sludge materials were the following: Calibration curve: a coefficient of determination (R 2 ) of at least using a linear curve. A set of seven standard solution prepared freshly on every measurement day with Cr, Cu, Mn and Zn with concentrations of 0,05 mg/l, 0,1 mg/l, 0,25 mg/l 0,5 mg/l, 1 mg/l, 5 mg/l and 10 mg/l were used for construction of the calibration curves. Linearity was determined via repeated measurements of the standard solutions. Maximum allowed RSD on replicates (not for blanks): less than 3 %. Maximum and minimum allowed blank levels: between -0,01 mg/l and 0,01 mg/l. Instrumental Limit of Detection (LOD) and Limit of Quantitation (LOQ): LOD < 0,02 mg/l; LOQ < 0,05 mg/l. LOD and LOQ were determined via the standard deviations of method blanks. Minimum and maximum allowed blank levels were established in the same experiments. Working range: between 0,05 and 10 mg/l for the calibration solutions. Repeatability (standard deviation): less than 5 % - Repeatability and intermediate precision were determined by replicate analysis of a CRM and assessment of between-day effects (Figure 1). Trueness to be able to measure the certified value of a CRM within combined uncertainty. Trueness, accuracy and method bias were checked against the certified value of the total trace element contents in the BCR 144R. Relative combined uncertainty maximum 10 % Uncertainty estimation The experimental design according to ISO [17] was set up in a way so that the repeatability, reproducibility and trueness estimates are used for measurement uncertainty estimation. Details on calculations are described elsewhere [18-20]. Trueness was proven by measurement of three independent samples of a CRM on two different days. From these data, the uncertainty of trueness and method bias were calculated. Repeatability and intermediate precision were determined by replicate analysis and assessment of between-day effects. This was achieved by preparation of three independent samples of the CRM on three extra days. Combination of these data and the data obtained for trueness were used for calculation of the uncertainties of repeatability and due to intermediate precision using the ANOVA function in the Microsoft Excel. Characterization set-up according to ISO Guide 35 [21] The characterization of the sediment materials has been carried out using a laboratory inter-comparison approach. The participants received two bottles of ERM-CC144 and were requested to provide 6 independent results, obtained on two different days. As a quality control measure, the participants also received a bottle of the BCR-144. One result had to be provided for this sample. RESULTS AND DISCUSSION Method development The digestion is of a critical importance for the measurement of trace elements in soil, sediment and sewage sludge materials. According to the microwave ultraclave manufacturer, hydrochloric acid (HCl) was not to be used in any sample preparation, which limited digestion options, since HCl is an acid of choice for soil, sediment and sludge materials digestion. When concentrated HNO 3 and HF were used in combination with microwave heating under high pressure, a residue was formed each time the samples were diluted with water prior to ICP-OES and ETAAS measurements (Figure 1), giving erroneous data for total Pb content in sediment materials [22]. Problems in the digestion of soil, sediment and sewage sludge materials were tackled to avoid the occurrence of residues in the solution. The occurrence of such a residue suggests incomplete digestion of the material and leads to poor recoveries for most of the trace ele- 19

4 Figure 1. Residue formation in the order sewage sludge, sediment, soil. ments, high RSDs for the measured elements, as well as partial blockage of the ICP-OES nebulizer. Different acid mixtures (concentrated acids were used throughout) were tested in combination with microwave heating under high pressure. The used sample intake was varied along with the acid mixture composition and volume, ranging from 0,1 to 0,5 g. Intake of 0,1 g was found to be the most suitable. The volume of the concentrated acids was varied from 2 to 10 ml (HNO 3 ); from 1 to 5 ml (HF); from 0,1 to 1 ml (HBF 4 ) and from 1,5 to 3 ml (H 2 O 2 ), with H 3 BO 3 stoichiometrically calculated to be 2,5 g. The ratio of the acid mixtures HNO 3 + HF and HNO 3 + HBF 4 was varied from 5+1 to 1+3, whereas for HNO 3 + HF + H 3 BO 3 and HNO 3 + HF + H 2 O 2 the ratio of the acid mixtures was varied from 3+1 to 1+3 with H 3 BO 3 kept constant (added before or after the digestion) and from to in the latter case. The residue formation was observed each time the samples were diluted prior to ICP-OES and AAS measurements. Instead, a diluted acid mixture of 20% v/v HNO 3 (kept constant) and HF (between 5% v/v and 20% v/v) was tested and the situation was improved the sewage sludge materials were completely digested and sediments were partially digested but with a considerably lower amount of residue formed. The method was found inapplicable to soils, which were not completely digested. The reason might be that the dilute acid mixture and the absence of HCl is not sufficient to break the silicate matrix for trace element release. A mixture of 10 ml containing 5 % v/v HF and 20 % v/v HNO 3 was found to be most suitable. The recoveries for all investigated elements improved significantly, and measurement RSDs were reduced (Table 2). The same approach, yielding 90,0±3,2% recovery was successfully used for the homogeneity, short-term and long-term stability studies of Pb in sediment materials [23,24]. Poor recoveries for the soil CRM when diluted and concentrated acid mixtures were used for the material digestion led to the validation only of a method for trace element determination in sewage sludge materials. Table 2. Comparison between the recoveries and their RSDs when concentrated and diluted acid mixtures were used for the digestion of soil (BCR-141) and sewage sludge (BCR-144) materials. Method validation During every measurement, 2 different emission lines were measured for all elements to calculate the concentration in solution, and the control line was used to check for interferences. The difference between the results of the 2 lines for Mn and Zn was always below 5%. For Cr, a few data points were recorded with a difference between the lines of between 5 and 6%. Although these points were higher than the 5 % target of the validation plan, it was still considered acceptable to use Cr 205,5 nm as the control 20

5 line, as the deviation was in the same order of magnitude as the repeatability of the method. For Cu, previous analyses of aqua regia extracts of soil and sewage sludge samples showed interference on the first, third and fourth most highly recommended lines [25]. Therefore, as part of this validation, an attempt was made to use the Cu line at 199,9 nm as an alternative. However, results showed an 8% difference from the measurement line, which could indicate interference on one or both of the measured lines. As the trueness experiments gave results with no significant bias (see below), which indicates that there is no interference on the measurement line (324,7 nm), it was decided to monitor the measurement line only, as a suitable confirmation line could not be found for this sample type (Table 1). A visualisation of the calibration lines and the residuals show no trends or deviations over the whole calibration range, but in the range of lowest standards (0,05; 0,1 and 0,25 mg/l), there is a visual trend. The RSDs of the higher part of the calibration curve (0,25 to 10 mg/l) are sufficiently similar to allow the use of relative uncertainty over this calibration range. However, in the lower part of the calibration curve, between 0,05 and 0,25 mg/l, the RSDs differ significantly, especially for the lowest standards 0,05 mg/l and 0,1 mg/l. As this would result in a change in the uncertainty due to repeatability, if the concentration to be measured were to fall within this range, it was decided not to include those standards into the calibration. The working range for the method was therefore set between 0,25 mg/l and 10 mg/l. Maximum and minimum allowed levels of the element concentrations in the blank solutions are defined to check for contamination of the samples or the calibration standards. The calibration was found not reliable when measuring samples below lowest standard. As a consequence, the minimum and maximum allowed blanks are set to 0,1 mg/l (Table 3). For the estimation of trueness, accuracy and method bias, as well as the repeatability and intermediate precision, three independent samples of BCR-144R were prepared on two different days and measured. From these data, the trueness and bias were calculated. Three independent samples were prepared on three extra days. From these data, repeatability and intermediate precision were calculated. Total of 6 sub-samples of the CRM have been measured on two different days to incorporate the reproducibility in the trueness check. Additionally, 3 subsamples of the CRM were measured on 3 extra days (5 different days, using 5 different calibration curves in total). The results were used to calculate the repeatabilites and the uncertainties due to repeatability and intermediate precision, based on ANOVA-single factor calculation. The estimation of trueness uncertainty was calculated to be higher when based on measurement repeatability for all cases in which no bias was observed (Cd, Cr, and Mn), and these are taken as the most conservative estimates. For Cu and Zn; while the uncertainty estimated due to bias was low compared to the total combined uncertainty, the trueness uncertainty for these elements is based on the observed bias, as a precaution. For Cu only, the targeted repeatability was not achieved. However, the experimentally obtained value was used in the calculation of the uncertainty [18-20, 22, 24]. The calculation shows that the expanded uncertainty for Cu is in the same range as the other elements and is in the same range as anticipated. Therefore, the obtained repeatability of the method is still considered fit-for-purpose. The relative expanded uncertainty was estimated (Table 4). Sewage sludge certified reference material characterization The fully validated method for the microwave-assisted digestion and subsequent determination of total Cr, Cu, Mn and Zn content using ICP-OES in sewage sludge samples was used within the characterisation exercise of ERM-CC144 (sewage sludge). The characterisation was done using an inter-laboratory comparison approach. The results obtained for the different elements in the quality control sample (BCR-144R) are summarised below: 21

6 Table 3. Targeted and achieved method performance criteria for the microwave assisted digestion and determination of Cr, Cu, Mn and Zn in sewage sludge CRM using ICP-OES Table 4. Uncertainty estimation for the determination of Cr, Cu, Mn, and Zn in sewage sludge material 22

7 Table 5. Measured and certified values for BCR-144R All the data were plotted together with the results obtained from the other participating laboratories. The results are summarised in Figure 2. Data from the validated method appears as L0 is the in the plots. CONCLUSION Based on the method validation results, the microwave-assisted method Figure 2. Summary of the characterisation data (normalised to the average of the study) for Cr (a), Cu (b), Mn (c) and Zn (d) for sample digestion and subsequent determination of Cr, Cu, Mn, and Zn in sewage sludge materials using ICP-OES, is considered under statistical control. It is evident that the in-house developed and validated method for the determination of trace elements in sewage sludge materials provides good comparability of the results between laboratories. It also has the potential to be applied when participating in PT-schemes. The method was successfully applied for homogeneity, short-term and long-term stability of total Pb content in sediment reference material BCR-320R and within interlaboratory comparisons when determining total trace element contents in sewage sludge reference material ERM-CC144 [26]. REFERENCES 1. Directive 86/278/EEC, 1986, Protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture, OJ L, 181: Quevauviller, Ph. Operationally defined extraction procedures for soil and sediment analysis. Part 3: new CRMs for trace-element extractable contents. Trends Anal. Chem., 2002, 21(11): Ignatowicz, K. The impact of sewage sludge treatment on the content of selected heavy metals and their fractions. Environmental Research, 2017, 156: Kazi, T.G. et. al., Evaluating the mobility of toxic metals in untreated industrial wastewater sludge using a BCR sequential extraction procedure and a leaching test. Anal. Bioanal. Chem. 2005, 383(2): ISO Soil quality extraction of trace elements soluble in aqua regia. 23

8 6. ISO Soil quality Microwave-assisted extraction of the aqua regia soluble fraction for the determination of elements. 7. Teixeira, L.S., Vieira, H.P., Windmöller, C.C., Nascentes, C.C., Fast determination of trace elements in organic fertilizers using a cuphorn reactor for ultrasound-assisted extraction and fast sequential flame atomic absorption spectrometry, Talanta, 2017, 119: Shirdam, R., Modarres-Tehrani, Z., Dastgoshadeh, F., Microwave assisted digestion of soil, sludge and sediment for determination of heavy metals with ICP-OES and FAAS. Rasayan J. of Chem., 2008, 1(4): Barałkiewicz, D., Hanć, A., Gramowska, H., Simultaneous determination of Cd, Cr, Cu, Ni, Pb and Zn in sewage sludge by slurry introduction ICP- OES method. Int. J. Environ. Anal. Chem., 2010, 90(14): Sandroni, V., Smith, C.M.M. Microwave digestion of sludge, soil and sediment samples for metal analysis by inductively coupled plasmaatomic emission spectrometry. Anal. Chim. Acta, 2002, 468(2): Granadillo, V., et al. Flame AAS determination of total lead in soil sediments near highways in Maracaibo City, Venezuela. Atomic Spectroscopy, 2006, 27: Deng, J., Feng, X., Qiu X., Extraction of heavy metal from sewage sludge using ultrasound-assisted nitric acid, Chem. Eng. J., 2009, 152: Dugenest, S., et al. Chemical characterization of municipal solid waste incineration residue: Dissolution of elements with a microwave-dilute acids digestion technique compared to conventional methods. Analysis 1998, 26: Marinos, A., et al. Effect of acid treatment on the removal of heavy metals from sewage sludge. Desalination 215 (2007), Mario, H., et al. Microwave-assisted digestion procedures for biological samples with diluted nitric acid: Identification of reaction products. Talanta, 2009, 79: ISO/IEC General requirements for the competence of testing and calibration laboratories. 17. ISO Guidance for the use of repeatability, reproducibility and trueness estimates in measurement uncertainty estimation. 18. Venelinov, T. Comparison of ISO and ISO standards for measurement uncertainty estimation in water analysis. Ovidius University Annals Series: Civil Engineering, 2016, 18: Venelinov, T. Uncertainty estimation of the determination of chemical sum parameters in water. Accred. Qual. Assur. 2017, 15: Venelinov, T. Correction to: Uncertainty estimation of the determination of chemical sum parameters in water. Accred. Qual. Assur. 2017, 15: ISO Guide 35, Reference Materials Guidance for characterization and assessment of homogeneity and stability. 22. Venelinov, T., Certification of Total Lead Content in Certified Reference Materials The Case of BCR-277R, BCR-280 R, and BCR-320R. Water Affairs, 2018, submitted. 23. Venelinov, T., Held, A. The Additional Certification of the Pb Mass Fraction in BCR-320R (Channel Sediment), 2009, ISBN , ISSN Venelinov, T. Validation of a method for the determination of total lead content in sediment materials. Ann. Uni. Arch. Civil Eng. Geod., , 45(7): Nölte, J. ICP Emission Spectrometry - A Practical Guide, Wiley-VCH, Weinheim, 2003, p Santoro A., et al. The certification of the mass fraction of the total content and the aqua regia extractable content (ISO and ISO 11466) of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Zn in sewage sludge, ERM - CC144: certification report, 2016, ISBN (PDF), ISSN