The Suitability of Digestion Techniques in the Assessment of Certain Metals in Standard Reference Materials

The assessment of various digestion methods is of utmost importance in accurately determining the concentrations of elemental metals in soil. In this study, the dry digestion method utilizing a graphite furnace oven was compared to the wet digestion method employing a microwave. Four standard reference materials, namely Randwijk clay, Hengelo sandy soil, Halle sandy soil, and Herveld clay samples, were subjected to analysis using an inductively coupled plasma optical emission spectrometer (ICP-OES). The reliability of the data obtained was ensured by calculating the recovery and error percentage of the results for both digestion methods. The findings indicate that the microwave digestion method is superior for all elements in soil samples, except for calcium, chromium, and magnesium. On the other hand, the dry digestion method may be favored for calcium, copper, chromium, magnesium, sodium, nickel, and vanadium. A pair t-test statistical analysis was conducted to compare the two methods, revealing significant differences, except for calcium, chromium, copper, iron, potassium, and sodium, indicating a lack of agreement between the two methods, except for these specific elements.


INTRODUCTION
Acidic digestion procedures are employed to convert solid samples into liquid extracts, facilitating the quantification of overall or pseudototal concentrations of metallic elements in soils.This crucial process involves the release of metals from the solid matrix into the acidic solution during extraction.Such procedures are essential for the determination of metals using conventional techniques like inductively coupled plasma optical emission spectrometry or atomic absorption spectroscopy. 1merous acid digestion methods have been documented in the literature for the analysis of heavy metals in soils.These methods encompass a wide range, from mild attacks, such as aqua regia in an open system, to the utilization of hydrofluoric acid in a closed system, which is considered a complete digestion method for the breakdown of silicate matrices 2 .The digestion of samples stands as a major contributor to the uncertainty surrounding analytical results, owing to the significant variations in metal content obtained through different methods [3][4][5][6] .To ensure the comparability of data, it is imperative for regulatory agencies to standardize the method employed for determining metal concentrations in soils.
Dlamini et al. , 7 presented the optimization, validation, and application of microwave-assisted digestion and inductively coupled plasma mass spectrometry (ICP-MS) for the simultaneous determination of trace metals [boron (B), cadmium, cobalt, chromium, copper, molybdenum, nickel, lead, selenium, vanadium, zinc, and arsenic] in soils from sludgeland.
Naicker et al., 8 described the analysis of twelve trace elements in soil and sediment samples using microwave-assisted and ultrasonic-assisted digestion prior to analysis with inductively coupled plasma optical emission spectroscopy.Agnieszka, et al., 9 conducted a study in which they presented the findings of their investigation into the measurement of heavy metal concentration in soil.They employed two distinct methods for adding soil components into solution and utilized different laboratory techniques and types of measuring equipment.The first method utilized was the hot digestion of soil samples with a mixture of concentrated HNO 3 and HClO 4 , following the prior ashing of organic matter (referred to as the IUNG method).The second method involved a two-stage decomposition process, whereby soil samples were initially hot digested with an oxidizing acid (HNO 3 ) and subsequently with a non-oxidizing acid (HF) (referred to as the two-stage decomposition method).The concentrations of selected heavy metals (Cr, Cu, Fe, Mn, Ni, Pb, and Zn) were determined in solutions obtained through both digestion methods.
The aim of this investigation is to assess the efficacy of two digestion methods, specifically the dry digestion method and the wet digestion method, in the determination of heavy metals in four standard reference materials.

MATERIALS AND METHODS
The development of the digestion methods and the analysis of four certified reference materials (CRMs), namely clays and sandy soil, were undertaken.HCl and HNO 3 were analytical grade reagents and were used as received.

Samples
Four certified reference materials (CRMs) were procured for the purpose of this study.The specific details of these samples are provided below (Table 1).
Two digestion methods were utilized, namely graphite furnace digestion (dry ashing) and microwave digestion, as outlined by Greenberg et al., 10 .

Dry digestion
A quantity of 1.00 g of each CRM was weighed and placed into a 30 mL porcelain crucible.The crucible was then introduced into a muffle furnace and gradually heated to a temperature of 700°C, allowing the sample to ashed for a duration of 4 hours.Following this, the crucible was carefully removed and cooled in a desiccator, as described by Chattopadhyay et al., 11 .Subsequently, 10 mL of Aqua regia solution (consisting of a 3:1 v/v ratio of HCl to HNO 3 ) were added to the cooled ash and stirred until dissolved.Any undissolved fraction was allowed to precipitate and then filtered into a 25 mL volumetric flask.The resulting solution was diluted with deionized water to a final volume of 50 mL.

Microwave Digestion
For this method, a mass of 0.2 g of each sample was weighed and placed into a microwave Teflon vessel.The vessel, along with the sample, was inserted into the HTC safety shield.Subsequently, 3.8 mL of HNO 3 (65%), 5 mL of HCl (37%), 1 mL of HF 40%, and 5 mL of H 3 BO 3 5% were added to the Teflon vessel.The vessel was then sealed and inserted into the rotor segment, which was in turn introduced into the microwave cavity and connected to the temperature sensor.The total digestion method was loaded and, upon completion of the program, the rotor was cooled using water until the solution reached room temperature.The vessel was then opened and the solution transferred into a 100 mL volumetric flask, following the protocol outlined by Ahmed et al., 12 .

Sample Analysis
The prepared samples from section 2 were subjected to analysis using inductively coupled plasma optical emission spectrometry (ICP/OES), employing the specified operational conditions showed in Table 2.

RESULTS AND DISCUSSION
The accurate determination of heavy metals in soils holds significant importance in the process of remediation of contaminated soils and the monitoring of land application of nonhazardous materials containing metals.Prior to the measurement of metal concentrations in soils, sample digestion is often required 13 .

Quality control of the two digestion methods
In general, the microwave digestion method applied to all four SRMs yielded accurate results (80-120% Recovery) for all elements, except for Mg and Cr in the Hengelo sandy soil and Herveld clay samples 14 .
In comparison to microwave digestion, the dry digestion method demonstrated accurate results for Ca, Cu, K, Mg, Na, and V in the Randwijk clay sample (Table 3), Na, Ni, and V in the Hengelo sandy soil (Table 4), Cr and Cu in the Halle sandy soil (Table 5), and Cu in the Herveld clay.From the data presented in Table 3, it can be observed that method A, also known as the Dry digestion method, exhibits favorable outcomes in terms of recovery and error percentage for the elements Ca, Cu, K, Mg, Na, and V. Conversely, it demonstrates unsatisfactory recovery and high error rates for the remaining elements.On the other hand, method B, referred to as the Microwave digestion method, demonstrates commendable recovery and acceptable error rates for all the elements under investigation.Analyzing the dry digestion of the Hengleo soil sample, as illustrated in Table 4, it becomes evident that the dry digestion method proves to be effective in terms of recovery and acceptable error only for Na, Ni, and V. Conversely, the microwave digestion method exhibits satisfactory recovery and error rates for all the elements being studied, with the exception of Mg. evident from method A that the dry digestion method in the Halle sandy soil sample yields favorable recovery and error rates only for Cr and Cu.In contrast, the microwave digestion method (Method B) demonstrates good recovery and error rates for all the elements under investigation.For the Herveld clay sample, the quality control results are displayed in Table 6.
In this case, the dry digestion method (method A) exhibits satisfactory recovery and error rates solely for Ca and Cu.On the other hand, the microwave digestion method (method B) demonstrates good recovery and error rates for all the elements under scrutiny, with the exception of Cr and Mg.
Overall, the quality control results for both the dry digestion method and the microwave digestion method indicate that the latter offers superior recovery and error rates.The ashing process in the dry digestion method is challenging to regulate and presents the potential for uneven heating and cross-contamination of samples 15 .

Comparison between the dry and microwave digestion methods
To conduct a comparison between the dry digestion and microwave digestion methods employed for the preparation of four Standard Reference Materials (SRMs) in order to determine the concentration of eleven elements using Inductively Coupled Plasma Optical Emission Spectrometry (ICP/OES), a paired t-test was used.The paired t-test was implemented using the equations provided by Shigeki Tsuneya et al., 16 , denoted as equation 1 and equation 2. Consequently, the paired t-test results for the dry and microwave digestion methods were obtained for the four SRMs using equations 1 and 2. These results were then tabulated in Tables 7-10, which display a comparison between the dry and microwave digestion methods.As demonstrated in Table 7, and based on the tabulated values for the two-sided t-distribution, the values for t 0.05 (t=1.8) and t 0.025 (t=2.2) exhibit no significant disparity.This indicates that both procedures are in concurrence with one another.As depicted in Table 8, and based on the tabulated values for the t-distribution with two tails, it can be observed that for t 0.05 (t=1.8) and t 0.025 (t=2.2),there is no statistically significant difference between any of the elements.This implies that both methods are in agreement with each other.As evidenced by the data presented in Table 9 and the corresponding values for the two-sided t-distribution, it is noteworthy that t 0.05 (t=1.8) and t 0.025 (t=2.2) exhibit statistically significant disparities across all variables, with the exception of Ca.This discrepancy implies that the two methods employed do not concur, except in relation to Ca.As presented in Table 10, and according to the tabulated values for the two-sided t-distribution, it is observed that for t 0.05 (t=1.8) and t 0.025 (t=2.2),all elements exhibit significant differences, except for Ca, Cr, Cu, Fe, K, and Na.This implies that the two methods do not agree with each other, except in the case of Ca, Cr, Cu, Fe, K, and Na.
Turek et al., 17 , conducted an investigation on various digestion procedures, namely drying and microwave digestion, ignition and microwave digestion, and drying and conventional digestion, for the purpose of evaluating heavy metal content in sludge samples.The results obtained indicate that the most effective method was ignition and microwave digestion.
Microwave-assisted and ultrasonic-assisted digestion techniques were applied prior to analysis using inductively coupled plasma optical emission spectroscopy, as described by Naicker et al., 8 .The authors concluded that both digestion methods yielded similar levels of accuracy, suggesting their suitability for precise determination of the target metals.Dlamini et al., 7 validated the microwave assisted digestion method for determining heavy metal concentrations in soil from sludge land, using ICP/MS.Their conclusion indicates that all validated parameters fell within acceptable limits, indicating the suitability of the method for its intended purpose.Abegunde et al., 18 conducted a study comparing three conventional acid digestion procedures for soil samples.Their findings revealed that the behavior of each metal towards the digestion acid can guide the selection of the appropriate digestion procedure.In an inter-laboratory study carried out by Santoro et al., 19 , different digestion methods were employed for a sewage sludge certified reference material.The results obtained demonstrated no significant differences between the extraction methods used.Two digestion techniques were compared and applied to real soil samples and standard reference materials for the analysis of Sb using ICP/MS.The recoveries of Sb achieved through HF in the acid digestion mixture in a closed-vessel microwave digestion system were found to be excellent, and the concentrations obtained were in very good agreement with certified or reported concentrations of reference materials.Monlau et al., 21 focused their study on anaerobic digestion with pyrolysis in soil.The results obtained indicated that both solid-digestate and pyrochar exhibited favorable properties as soil amendments, albeit with complementary effects.

CONCLUSION
The comparison of dry digestion method and wet digestion method show that the microwave digestion method was superior for analyzing all elements in soil samples, except for calcium, chromium, and magnesium.On the other hand, the dry digestion method is recommended for the analysis of calcium, copper, chromium, magnesium, sodium, nickel, and vanadium base on the present study.Based on the obtained results, the conclusion is drawn that microwave digestion is suitable for sample preparation for various soil types, including soil, sandy soil, and clay samples.The microwave digestion method is recommended for use in determining heavy metal concentrations in soil and river basin samples.This recommendation suggests that the microwave digestion method is reliable and effective for preparing samples and extraction of heavy metal content in environmental samples.
represents the calculated t-value, d represents the mean difference, Sd represents the standard deviation of the difference, SE(d) represents the standard error of the mean difference, and N represents the number of readings.

Table 5
presents the quality control outcomes for the Halle sandy soil sample.It is

Table 7 : Randwijk clay sample comparing the dry and microwave digestion methods by mean of paired t-test
Element Difference (µg/g) standard deviation (µg/g) SEof the mean difference Calculated t