Characterization of Different Types of Biomass Wastes Using Thermogravimetric and ICP-MS Analyses

LIDIA KIM, GINA ALINA CATRINA (TRAISTARU)*, CRISTINA ILEANA COVALIU LUOANA FLORENTINA PASCU, IONUT CRISTEA, AGNES SERBANESCU, GEORGIANA CERNICA National Research and Development Institute for Industrial Ecology ECOIND, 71-73 Drumul Podu Dambovitei, 060652, Bucharest, Romania University Politehnica of Bucharest, Faculty of Biotechnical Systems Engineering, 313 Splaiul Independentei, Bucharest, Romania


Experimental part
In study were selected and analysed three types of biomass wastes such as: S1-vegetable waste from food industry; S2 -wood waste from forestry; S3-vegetable waste from farming.
The samples were dried at 105ºC and crushed less than 1mm. The moisture content was approximately 3 % -3.7% for all the analysed samples.
Before metal analysis a calcination process was applied, the biomass samples were thermal treated with two different methods, such as: calcination at 550ºC (method I), respectively at 820ºC (method II) for 1h in a calcination oven.

Materials
All the chemicals were of analytical reagent grade (Merck quality). Ultrapure water was produced with a Millipore Milli-Q System. For the calibration curves were prepared standards in the range 20 to 100 µg/L using a 1000 mg/L Multielement Certified Reference Material (ICP multielement standard solution XXI, Certipur, Merck). Quality control of the analytical results was performed with a multielement Certified Reference Material for ICP, 100 mg/L, Merck quality.

Equipment
Microwave Analytical method (i) Digestion procedure 0.5 gram of ash sample was introduced into the digestion cell and 10 ml of nitric acid (65%) was added. After the digestion was completed, solution from the digestion cell was filtered through filter paper (porosity 0.45μm) and brought to a volume of 25 mL, then the samples were analysed by ICP-MS equipment. The ash samples obtained with both calcination methods were treated in same conditions. The digestion of samples was performed with a Microwave Digestion System Ethos Up Milestone following a program presented in Table 1. (ii) Metal analysis The metal (As, Cd, Pb) concentrations were determined using a mass spectrometer with coupled plasma (ICP-MS) type 7900 Agilent with Mass Hunter 4.4 software. The ICP-MS operating parameters are presented in Table 2. The in-house validation of the analytical method applied was performed in order to evaluate the performance parameters of the method: limit of detection LOD, limit of quantitation LOQ, repeatability, intermediate precision, expanded uncertainty and recovery. The experimental studies (table 3) were applied on S2 sample due to high content of metals. The expanded uncertainty of the analytical results was estimating using following formula [18]: where k is a coverage factor; value 2 for 95% confidence level; Ucombinedcombined standard uncertainty; Ucconcentration uncertainty (instrument calibration, flasks, pipettes, reference standard material); Uv -50-ml volumetric flask (calibration, temperature); Ureprepeatability uncertainty (mass, volume, concentration, extraction recovery); Umassweight uncertainty (balance calibration, linearity); Urecextraction recovery uncertainty.

(iii) TG analysis
To evaluate the structural characterization and behaviour of the biomass waste it was use a thermogravimetric analyser Netzsch STA 409, TG operating parameters being presented in table 4, while the mass transformation during the thermogravimetric analyses is presented in table 5.  Decomposition of proteins 320-450ºC 4 Decomposition of volatile substances and the carbonates >500ºC

5
Char decomposition, responsible for the late formation of H2 and CO. >750ºC

Results and discussions
Thermogravimetric analyses Thermogravimetric analysis describes the processes of decomposition of matter under the influence of temperature. At the end of TG analysis, it is known the temperature where the sample was decomposed and the ash content obtained for each samples.
In the study, the biomass wastes undergoes various transformations of weight depending on their chemical composition: processes of dehydration, carbonization and combustion [1], the changes of waste's weight being described in Figures 1-3.
The degradation thermal step is influenced by temperature and the chemical composition of the biomass wastes. The TG analysis of S1 sample (vegetable waste from food industry) is presented in Figure 1, the mass transformation during the analysis being presented in table 7.  The residual mass (ash) left in the alumina crucible at the end of the assay was 13.48% ( Figure 1). Complex matrix of vegetable waste from food industry is composed from: peel potatoes, peel tomatoes, scrap processing vegetables etc. This complex matrix contains volatile substances and carbohydrates which decompose as it can see in Table 6.
The TG analyze of the S2 sample (wood waste from forestry) is presented in Figure 2. The wood waste from forestry contain: sawdust from woodworking, sawdust from wood, woody debris, branches, bark etc. The thermal degradation process for S2 sample is presented in Table 8.  The residual mass (ash) was 26.82% ( Figure 2). After the decomposition of volatile substances and the carbonates, the S2 sample still has a high decomposition char of approximately 27% at 940ºC (table 7).
The TG analyze for the S3 sample (vegetable waste from farming) is presented in Figure 3. The S3 sample is composed from fruits and vegetables wastes.  The main constituents of the biomass wastes are: cellulose, hemicellulose and lignin. The wood waste from forestry decomposes more slowly than vegetable waste from food industry and vegetable waste from farming, as it can see in the table 6 to 8.
From the table 7 it can be concluded that the S2 sample contains 97.92% solid material that must decomposed under the thermal treatment. The amount of ash resulting from thermogravimetric analysis for the three samples analysed decreases in the following order of: S2 (26.82%)> S1 (13.48%)> S3 (5.86%).
The measurement results for TG analysis show high volatile content (35.58%) for S1 sample and low volatile content (7.30%) for S3 sample. The S2 sample presents 30.68% volatile substances as it can see in table 7. Thermal analysis has shown to be a powerful tool for investigating the thermal degradation processes from our biomass wastes.
If we know the behaviour of the biomass waste under the thermal degradation process, we could estimate the amount of pellets or briquettes that are necessary to heat a home. From the residual ash of our biomass wastes sample we could estimate the quantity of metals that remains after the combustion step. These processes are highly important, given the risk of fouling and ash slagging on the walls of home heated surfaces.

Determination of the As, Cd, Pb in the biomass ash
In table 9 are presented the values of performance parameters obtained in in-house validation experiments using applied analytical method (ICP-MS technique). The results of As, Cd and Pb for both thermal treated methods are indicated in Figure 4. The differences between three biomass wastes include the moisture content and chemical composition of the biomass ash wastes, which will have a potential effect on the plant combustion efficiency. The biomass coming from the wood waste has small moisture content (2.95%), but high content of metals. This type of biomass waste could be a promising materials for the future valorisation as pellets or briquettes for combustion plant, but also for domestic use.
The content of metals in biomass ash is variable and depends on the type and origin of biomass, harvesting time, sampling technique, transport and storage. a) b) c) In the S2 sample ash it was obtained a higher metal concentrations compared to the other two samples as shown in Figure 4a) and b). As it could be observed in Figure 4a) the arsenic highlighted a higher concentration for the S2 ash sample. In figure 4c), the lead highlighted a higher concentration for the S2 sample coming from the wood waste. The results obtained using calcination method II conduct to higher concentration of metals for all samples.

Conclusions
The content of toxic elements As, Cd and Pb was determined in three biomass waste samples resulted from different activity sectors from Romania. The decomposition of biomass waste using TG analysis and the amount of the ash gives us valuable information reusable biomass wastes, being potential materials for valorisation as pellets or briquettes. Biomass sample calcination at 550 º C can give inaccurate results from ICP-MS analysis as result of significant unburned carbon content, retained in the ash mass. Processing the sample at a temperature of 820 º C produces an ash composition more suitable for ICP-MS analysis. Good and reproducible results were obtained follow method II (820ºC) for all the toxic elements analysed. The biomass waste indicate for S2 sample highest concentration of metals and ash content. The recovery percentage were higher that 84%, the highest recovery yields (>90%) was obtained for cadmium and lead.
As a conclusion, method calcination at 820 º C is suitable for the characterization of the toxic elements As, Cd and Pb from the biomass wastes.