Sonoleaching: Development of a rapid determination of Pb ... - SER

More documents

Recommendations

Info

376 Bellotindos et al., Sustain. Environ. Res., 21(6), 375-380 (2011) the results are not valid for the purposes of determining whether the waste is hazardous based on toxicity characteristic. . In the Philippines, Congress enacted many laws to address the management and control of increasing volume of wastes produced. One of these laws, the Toxic Substances and Hazardous and Nuclear Wastes Control Act of 1990 (Republic Act 6969), was enacted in response to the increasing problems related to toxic/hazardous chemicals and nuclear wastes in the country. The implementing rules and regulations of this Act require the use of TCLP. . Rossi [3] described leaching as a complex dissolution process that requires suitable reactants in aqueous solution to come in contact with the mineral particles to be dissolved. TCLP estimates the extent of leachability of hazardous constituents from solid wastes under certain conditions. This procedure consists of many components, one of which is the extraction with simulated leaching fluid using a rotary apparatus for 18 h. This, in addition to the time for sample preparation before extraction and the final separation of liquid and solid phases for final analysis of the target constituents, is time consuming. The total time for a TCLP determination can reach 22 h. . This limitation of TCLP may be addressed by replacing the 18-h contact time in a rotary extractor with ultrasound assisted extraction. Ultrasound is the sound with frequency beyond the human hearing threshold [4]. The human hearing frequency is normally 16 to 18 kHz. The use of ultrasound in extraction has been gaining popularity in the past decade although researches on ultrasonics started in the late 1920s and into 1940s [5], but interest on the application of ultrasound dates back more than 100 yr [6]. Ultrasound is a useful tool in enhancing reaction rates in many reaction systems. This rate enhancement is called sonochemistry. . Sonochemistry, which is the use of ultrasound to enhance or alter chemical reactions [6], began in the late 1800s. However, as mentioned by Thompson and Doraiswamy [6] in their review paper on sonochemistry, the term was first used by Neppiras in 1980 in his review of acoustic cavitation. Chemical effects of ultrasound include improved conversion and yield, change in reaction pathway or initiation or reactions in chemical, biological or electrochemical systems. Ultrasound also results in physical effects which include increasing the surface area of the reactants and accelerating dissolution. . The driving force in sonochemistry is cavitation; the formation, growth and implosive collapse of bubbles in a liquid [7,8] which generates heat and produces intense local heating (5000 °C) and high pressure (200 MPa). This type of cavitation resulting from the application of sound waves is known as acoustic cavitation [9]. The effect of cavitation within the liquid depends on the type of system where it is generated. In the case of a solid-liquid system, cavitation collapse near a particle can lead to shock waves that can break the particle apart or force it into rapid motion. These result in interparticle collisions that cause erosion, wetting of the particles, surface cleaning and particle size reduction [4]. Many studies have been conducted on the various applications of ultrasound including the effects of ultrasound assisted extractions of metals from soils, sediments and solid wastes. In these studies, researchers found that ultrasound provided savings in extraction time. Collasiol et al. [10] developed and established a method for mercury extraction in sediment and soil using ultrasound and results showed that the method was fast and mercury loss was prevented. The leaching yield of the silver content of a mining waste was investigated by Öncel et al. [11]. The experiment showed that silver may be leached almost completely from the solid waste of a silver ore plant by means of ultrasound assisted thiourea leaching method. Al-Merey et al. [12] investigated the experimental conditions of an ultrasonic cleaning bath for quantitative extraction of lead, copper and zinc metals from soil samples. The results showed that the performance of the method was equal to a hot-plate digestion method and significantly reduced the hazardous and fumehood emissions. Mason et al. [13] reviewed current laboratory research and potential for the scale-up of chemical decontamination using ultrasound and concluded that the use of ultrasound in the laboratory cleaning of soil samples proved to be effective and some large scale trials showed promise. Meegoda and Perera [14] in an attempt to develop a technology to decontaminate heavy metals in dredged sediment using ultrasound coupled with vacuum pressure concluded that although the clay fraction could not be effectively treated by this technology, chromium was immobile in the clay fraction of the treated sediment and was safe for disposal. Marin et al. [15] developed a method for determination of zinc and arsenic speciation in soils using focused ultrasound. The method, which replaced extraction with mechanical shaking by sonication to simplify analytical procedures was optimized and validated. In a study by Perez-Cid et al. [16], the use of focused ultrasound was applied to a sludge sample to shorten the operation time in each of the stages corresponding to a sequential extraction method proposed by the Community Bureau of Reference (BCR). The sonication conditions (sonication power and time) were optimized and extraction of copper, chromium, nickel, lead and zinc was compared with the conventional three stage sequential extraction method. The use of ultrasound represented a valid alternative to the conventional shaking and reduced the operation time. In a similar work, the use of ultrasound provides a saving in extraction time relative to a conventional mechanical shaking as shown by Kazi et al. [17] who developed a rapid version of the three-stage BCR sequential extraction to release heavy metals from . . . . .
untreated waste samples. The use of ultrasonic bath offered the advantage of replicate extractions that can be carried out simultaneously. Munoz et al. [18] proposed a combination of ultrasonic extraction and stripping technique (anodic stripping voltammetry) for routine analysis of copper and lead determination in lubricating oil samples. The method which used an ultrasonic bath provided the advantage of simultaneous sample pretreatment. Bellotindos [19] showed that sonication is better than extraction by TCLP on extraction of Pb from synthetic soil. Table 1 lists some studies on Pb leaching by various sonoleaching operation (frequency and time). . Table 1. Some studies on ultrasound assisted extraction of Pb Sample BCR certified reference sediments Extraction time (min) 5-60 Frequency (kHz) Edible seaweed 10 35 Sediments 5-35 50-60 To determine the effects of two or more factors or how these factors interact with each other, experiments are performed. In the past, the one-factor-at-a-time approach, where one factor is varied over the range of given condition while keeping all other factors constant, was practiced. This method is time consuming and does not consider the interaction between factors [23]. The Design of Experiment is a planned approach in determining cause and effect relationship [24] using inputs and outputs that can be measured. This approach has become a strategy of researchers to shorten the time of the study in terms of the number of experimental runs and to save on expensive trials. To optimize multivariate systems, statistical software packages such as Design-Expert 7 are used by researchers. Optimization is achieved by Factorial Design analysis [25,26], Fractional Factorial Design [27,28] or Re- sponse Surface Methodology [29]. There is a growing concern for the increasing amount of Pb as a contaminant in the environment and as an environmental health issue. It poses great risks to human health due to its proven toxicity. This research explored the use of an ultrasound assisted leaching for a rapid determination of Pb from stabilized waste. In this study, stabilized waste from slag was made to find out the effects of ultrasound in extracting the metal and comparing it with TCLP extraction and to find out the mechanism of extraction with ultrasound. The optimal conditions comparable to TCLP extraction were determined. . MATERIALS AND METHODS 1. Chemicals and Analytical Methods . Bellotindos et al., Sustain. Environ. Res., 21(6), 375-380 (2011) % Pb extracted 48-54 0.6-1.4 -1 (µg g ) 7.6 -1 (µg g ) Reference Canepari et al. [20] Dominguez- Gonzalez et al. [21] Elik [22] . The water for all solution preparations was ultrapure water (18.27 MÙ cm at 29 °C) supplied by RODA ultrapure water systems. Pb was analyzed using PerkinElmer AAnalyst 200 Atomic Absorption Spectrophotometer. The rotary agitator for the TCLP was supplied by Cherng Huei Co. (Taiwan), Model RA- 326, 40 W, 110 V with timer and rpm control. The temperature feature of the Suntex conductivity meter was used to measure temperature. . 2. Sonoleaching Set-up The source of ultrasound was DELTA Ultrasonic Cleaner (tank capacity = 10.8 L, operating frequency = 40 kHz). The ultrasonic bath has a temperature and time control which can be changed in increments of 1. The reaction flask is suspended by the use of a clamp and the level of the liquid in the reaction bath is kept at the level of the water bath. The bath temperature is monitored by the temperature feature of a Suntex conductivity meter. Figure 1 shows the actual picture of the set-up. . 3. Extraction Procedures 377 For extraction by TCLP, the USEPA SW-846 Method 1311 was used. For ultrasound procedure, distilled water was poured into the ultrasonic bath to a level about 5.1 cm from the rim and degassed for 5-10 min. An amount of the sample was weighed into a 250 mL Erlenmeyer flask. The extraction fluid (TCLP ex- Fig. 1. Sonication set-up. traction fluid) was added using the solid to liquid ratio of 1:20. Five flasks were prepared for the different ultrasound exposure time (15-90 min). The flask was suspended in the bath such that the level of the extraction fluid in the flask was at the same level as the water in the tank. A temperature probe was suspended at the same level as the bottom of the flask. The duration time was set and ultrasound was applied. The bath temperatures at the start and end of sonication were noted. The concentration of the Pb in the extract was determined by Atomic Absorption Spectrometer. . . .
Page 1: Sustain. Environ. Res., 21(6), 375-
Page 5 and 6: volume of the extraction fluid used

Sonoleaching: Development of a rapid determination of Pb ... - SER

Create successful ePaper yourself

Delete template?

Save as template?