Experimental method for the assessment and characterization of hazardous waste for long-term storage

Long-term landfilling of hazardous waste should be a careful choice for any producer or generator of waste, as the behavior of the waste is different due to the physical-chemical conditions or following contact with other hazardous waste. In this study, the research undertaken was aimed at developing an experimental method for the assessment and characterization of hazardous waste for long-term storage. The method consists in the assessment of the behavior of heavy metals from waste, under different leaching conditions. To study various hazardous wastes, fly ash from the incineration of medical waste and slag from aluminum casting were chosen. Contact time, pH and redox potential are important parameters in the leaching process. The solubility of metals increases at pH values between 2 - 5 pH units and decreases at pH values between 6-12 pH units. The highest solubility of metals (As, Cr, Cu, Ni, Pb and Zn) in the tested waste was obtained after 48 hours at pH values between 2 - 5 pH units. Also, the values of the redox potential decrease almost linearly as the pH value increases. The developed method is a useful tool to assess the behavior of hazardous waste for long-term storage in landfills for this category of wastes.


INTRODUCTION
Waste generated from industrial activities leads to environmental pollution with heavy metals. Because many wastes contain pollutants from the list of priority hazardous substances, disposal of such wastes should be the last option in the hierarchical order of waste management [1,2]. Simultaneously with the quality control activities of the deposited waste, the storage landfills must comply with the imposed standards so that the impact on the environmental factors and the human health is minimal [3,4]. Currently, does not exist a specific technology beeing 100% effective for stabilization of a particular waste type or waste class. The selection of the most appropriate technology is made following of waste characterization and based on laboratory experiments designed to establish the optimal operating parameters [5][6][7][8][9]. The laboratory experiments are based on leaching tests by the standardized method SR EN 12457: . The results are compared with the limits imposed by MMGA Order no. 95/2005 on establishing acceptance criteria and preliminary procedures for acceptance of waste for disposal [10][11][12][13][14]. The standard leaching method is based on the contact between the solid waste and distilled water or a specific solution for 24 hours. Through this method of leaching, the solubilization process of pollutants from waste is evaluated over time, being determined factors for selecting the type of landfill in which the waste can be deposited [13,14]. For the assessment of hazardous waste in long-term storage, the standardized method CEN/TS 14429: 2005 should be applied. The leaching test provided by this method is particularly important because it provides data on the composition and behavior of waste in long-term storage [15].
The pH values play a decisive role in stabilizing waste, because the solubility of metals depends on pH. In the case of waste containing metals with different solubilities at the same pH value, the minimum solubility of each metal species shall be taken into account when choosing the pH for the leaching test. The increasing pH leads to the precipitation of metals in the form of hydroxides with low solubility and implicity to obtaining the leachate with a low metals content [7][8][9][10]. The redox potential (ORP) is another important parameter considering that in the stabilization processes there are metals with multiple valence states (Cr, As, Se). In the case of hexavalent chromium (particularly toxic), it must be reduced to trivalent chromium (with ferrous sulphate or sodium bisulphite) which is easily immobilized by pH control. Thus, the influence of redox potential (ORP) must be studied before selecting the appropriate technology for the stabilization of metal-containing waste [21,22]. Increasing the pH to maximum values is not a 100% efficient solution for stabilizing metal species due to the complexity of the waste matrix. Carbonates, silicates, phosphates and other precipitatingcomplexing agents are used to stabilize waste containing heavy metals [22][23][24]. The experiments performed in this study aimed, on the one hand, to optimize an existing method, in the sense of its development, and on the other hand to implement the method in the current activity of our waste analysis laboratory, to meet those needs to assess dangerous wastes behaviors that are landfilling for a long time periods.

Reagents and calibration
Calibration curves for As, Cr, Cu, Ni, Pb and Zn were preformed using 100 mg/L Multielement Certified Reference Material, traceable to NIST, containing 23 elements. Quality control of the analytical results was performed with Multielement standard contains 21 elements purchased from VWR Chemicals. All the reagents used in the experiments were of analytical quality grade. In order to assess and develop the analytical method, two types of waste were chosen. A fly ash from medical waste incineration (6.35 pH units), noted P1 and a slag from the casting of aluminum (9.52 pH units), noted P2, were selected.

The method and samples preparation
The parameters that influence the leaching tests were: pH of solutions, contact time, solid / liquid ratio, chemical composition of the waste (metals content), particle size of the selected samples (≤ 1 mm or ≤ 4 mm), stirring speed and working temperature (20 ± 5°C). The amount of dry material and the amount of water to obtain the ratio of 1:10 were selected according to the indications in the standard SR EN 14429:2015 [15]. The volume of acid or of base that was added over the two types of wastes to obtain the desired pH was also selected according to the same standard. The method consists in testing the leaching of waste for a period of 48 hours under controlled conditions of time and pH (at t = 0; t = 30 min; t = 2h; t = 4h, t = 44h and t = 48h and pH between 2 -12 pH units for each set of time). The main steps for preparing the waste samples were the following: a) drying, sieving, weighing;

Dangerous waste
The initial physical-chemical and structural characterization of the waste Establishing the optimal conditions for the leaching test, ratio liquid: solid =10:1 L/kg, (correction of the initial pH of the sample using acid/base addition)  [14] b) determination of the acid / base quantity for leaching tests and correction of the samples pH between 2 and 12 pH units; c) shaking the samples for 48 hours at 100 rpm on a Shaker stirrer; d) analysis of specific metals using ICP-MS technique at different contact times between waste and leaching solution (t = 0; t = 30 min; t = 2h; t = 4h, t = 44h and t = 48h) e) statistical processing of the obtained results and validation of the method.
To control the quality of the leaching tests, extraction for a waste-free control sample was applied to each set of analyzes. The general flow scheme for the characterization and evaluation of some contaminants from waste is described in figure 1.

Diggestion step
After collecting the samples from the leaching tests, the extracts were filtered through a 0.45 μm Milipore filter and the liquid was mineralized using a closed microwave digestion system. For the digestion step, 2 ml of each sample was placed in a digestion vessel over which 7 ml of HNO3 and 3 ml of HCl were added. The microwave power was 1600 watts and the temperature was 150°C. After the digestion stage, the samples were analyzed by ICP-MS for the analysis of heavy metals.

Validation of the method
The method was in-house validated, by analyzing the 1: 10 leachate resulting after 48h, at the value of 12 pH units, for both types of samples. The tested parameters were detection limit (LOD), quantification limit (LOQ), eluting repeatability and leaching test repeatability. The ICP-MS calibration curve for six metals (As, Cr, Cu, Ni, Pb, Zn) was performed in the range of 20 µg/L to 100 µg/L.

RESULTS AND DISCUSSIONS
Particle size distribution and specific surface area The particle size was adjusted to assess the hazard time of the deposited waste (grinding to size les than 1 mm). The morphological analysis was performed to highlight the particle size distribution for each sample and to correlate the metal content with the particle size. The smaller the particles, the more accurate the metal leaching test for 48 hours was. Table 2 shows the values obtained for the specific surface area and the particle size distribution for the two types of waste.  As in the case of sample P1, for sample P2 the particle size distribution also changed very little (for 90% of the sample value they changed from the initial moment (0.951 µm) to the final moment (0.960 µm), insignificant values for the grinding tests (figure 3b). Based on the morphological analysis (particle size distribution and specific surface area), it was demonstrated that the samples were correctly prepared for the leaching tests.

In-house validation experiments
In table 3 are presented the values of the performance parameters obtained in the in-house validation experiments using ICP-MS technique for all the analyzed matrices. The method of evaluation and characterization of some contaminants from hazardous waste is precise, exact, presenting detection limits and small quantification limits for As, Cr, Cu, Ni, Pb and Zn by ICP-MS analitycal technique.

Determination of metals in various leaching conditions using ICP-MS technique
The calibration plot for metals were made in the concentrations range 20-100μg/L using ICP-MS technique. Maximum allowed concentrations (CMAs) according to Order 95/2005 [14] are presented in table 4. In Figure 4 (a) to (f) are presented the obtained results for different leaching conditions:   At the initial pH, P1 sample waste type could be stored in non-dangerous waste landfills and the P2 waste type could be stored in dangerous waste landfills (figure 5) according to Romanian in force legislation [14]. By changing the pH, the leachate of the tested wastes changed their chemical composition and implicitly the classification criteria for storage. At pH=12, P1 waste sample type could be storage in inert waste landfills, while and P2 waste type sample could be preserve in non-hazardous waste landfills ( figure 6).

Reducing and oxidizing conditions
In Figure 7 are presented the obtained results for Redox Potential (ORP) before and after treatment. The waste leachate could have both oxidizing and reducing properties when pH was adjusted in the range between 2-12 pH units. After 48 hours of contact between aqueous solution and solid waste it was observed a increasing tendency of the Redox Potential due to anions complexation. As a result of the composition changes, the Redox Potential changes its electric charge, presenting a negative value. After 48 hours, small variation of the pH were observed. The ORP value has an important role in the leaching process. The absence of oxygen leads to the formation of different chemical phases with significantly different solubilities compared to environmental conditions.

CONCLUSIONS
In the present study, a series of experiments were performed for the characterization and classification of two different types of waste, in accordance with Romanian Order 95/2005, simultaneously with an evaluation and optimization of the analytical method to highlight the longterm hazard of hazardous waste storage. Based on the obtained results, it can be concluded that the metal concentration is directly influenced by the pH value, the contact time and the redox potential of the leachate. As the pH value increases from 2 to 12 pH units, the concentration of metals decreases. A precipitation potential of all leachate anions and cations was also identified, which is highlighted by measuring the redox potential (ORP). The 24-hour leaching test for the initial pH of the waste is not sufficient to assess hazardous waste for long-term storage of waste. For these reasons, the 48-hour leaching test method has been developed and implemented at the laboratory level to assess the hazard of waste in the case of long-term storage. Both leaching tests (24h and 48h) should be used, together or consecutively, for better characterization and evaluation of waste under different environmental conditions. It is known that waste has characteristics and chemical composition that can change during landfilling conditions, the method developed and presented in this paper is a useful tool for characterizing hazardous waste.