Optimization of digestion methods for sewage sludge using the Plackett-Burman saturated design

The Plackett-Burman saturated factorial design was used to select optimized dissolution conditions for sewage sludge samples. Three different digestion methods were applied: i) microwave oven digestion in a domestic oven with Parr-type reactors; ii) microwave oven digestion with controlled-pressure reactors; iii) pressure bomb reactor heated on a hot plate. The three methods were validated by statistically comparing the metal contents found with the certified ones of the sewage sludge sample (BCR 145R). No significant differences were obtained and the RSD values were lower than 3% in all cases. The metals were determined by flame-AAS. The variables studied were the following: microwave power; digestion time; predigestion; volume of hydrochloric acid; volume of hydrofluoric acid; volume of nitric acid. The operative advantages offered by microwave digestion with controlled-pressure reactors were also considered. Received: 23 July 1997 / Revised: 31 October 1997 / Accepted: 5 November 1997     

Automatic flow-injection system for the determination of heavy metals in sewage sludge by microwave digestion and detection by inductively coupled plasma-atomic emission spectrometry (MW-ICP/AES)

Results are shown obtained in the optimization of an automatic flow injection system that combines microwave digestion with atomic spectrometric detection (FAAS, ICP/AES) for the determination of heavy metals in sewage sludge. Digestion is performed by preparing a suspension of the sample in 1.5 mol/l HNO₃ and making it flow through a PTFE capillary tube placed inside a conventional microwave oven. The effects of the length and inner diameter of the capillary tube, as well as that of the pumping rate, have been studied in order to find the experimental conditions that allow a quantitative elemental recovery in the shortest period of time possible. The optimization study was carried out on a certified sample (BCR No. 146), and the elements determined were Zn, Cu, Pb, Cd, Ni and Cr. The experimental data (percent recovery vs. digestion time) have been fitted to a mathematical model in order to quantify the influence of each of the variables studied. The optimized procedure (MW-ICP/AES) has been applied to one ordinary and one certified sewage sludge sample. In comparison with the conventional methods of sewage sludge analysis, the one proposed is less time consuming, while being equally precise and accurate.     

Automatic flow-injection system for the determination of heavy metals in sewage sludge by microwave digestion and detection by inductively coupled plasma-atomic emission spectrometry (MW-ICP/AES)

Results are shown obtained in the optimization of an automatic flow injection system that combines microwave digestion with atomic spectrometric detection (FAAS, ICP/AES) for the determination of heavy metals in sewage sludge. Digestion is performed by preparing a suspension of the sample in 1.5 mol/l HNO(3) and making it flow through a PTFE capillary tube placed inside a conventional microwave oven. The effects of the length and inner diameter of the capillary tube, as well as that of the pumping rate, have been studied in order to find the experimental conditions that allow a quantitative elemental recovery in the shortest period of time possible. The optimization study was carried out on a certified sample (BCR No. 146), and the elements determined were Zn, Cu, Pb, Cd, Ni and Cr. The experimental data (percent recovery vs. digestion time) have been fitted to a mathematical model in order to quantify the influence of each of the variables studied. The optimized procedure (MW-ICP/AES) has been applied to one ordinary and one certified sewage sludge sample. In comparison with the conventional methods of sewage sludge analysis, the one proposed is less time consuming, while being equally precise and accurate.     

Some observations on the determination of total heavy metals in sewage sludge by atomic absorption spectrophotometry after a pressurized acid digestion

An economical and simple method for the flame atomic absorption spectrometric determination of Cd, Pb, Ni, Cu, Fe, and Zn in sewage sludges is described. Samples are treated with concentrated HNO₃ in a thermal oven using Pyrex glass tubes hermetically sealed with Bakelite screw caps. The effect of the digestion parameters, such as digestion time, acid volume, sample mass, and temperature, were studied. The precision and accuracy of this procedure were evaluated by the analysis of two BCR (Community Bureau of Reference, European Communities) certified sewage sludge samples (CRM 146 and 144).     

On-line microwave-assisted digestion of solid samples for their flame atomic spectrometric analysis

A new procedure has been developed for the on-line digestion of solids in a microwave oven. The direct injection in a water carrier flow of dispersions of solid samples in concentrated nitric acid, the merging of these slurries with 30% (v/v) H(2)O(2) and the microwave-assisted digestion in a Teflon coil of 100 cm permit a fast and quantitative extraction of Cu and Mn from different solid matrices, such as vegetables, powdered dietary products and sewage sludges. The development of an appropriate interphase, in which digested samples are cooled and degassified, previous to their introduction into the nebulizer of a flame atomic absorption spectrometer, makes possible the full automatization of the digestion and measurement steps of the elemental analysis of solids and it provides a sample frequency of 180 injections per hour. The developed procedure has also been applied for Pb and Zn determination in certified sewage sludge samples, with accurate results obtained for Pb but low results found for Zn.     

Gas chromatographic-mass spectrometric study of the oil fractions produced by microwave-assisted pyrolysis of different sewage sludges

The pyrolysis of sewage sludge was studied in a microwave oven using graphite as microwave absorber. The pyrolysis temperature ranged from 800 to 1000 degrees C depending on the type of sewage sludge. A conventional electrical furnace was also employed in order to compare the results obtained with both methods. The pyrolysis oils were trapped in a series of condensers and their characteristics such as elemental analysis and calorific value were determined and compared with those of the initial sludge. The oil composition was analyzed by GC-MS. The oils from the microwave oven had n-alkanes and 1-alkenes, aromatic compounds, ranging from benzene derivatives to polycyclic aromatic hydrocarbons (PAHs), nitrogenated compounds, long chain aliphatic carboxylic acids, ketones and esters and also monoterpenes and steroids. The oil from the electric oven was composed basically of PAHs such as naphthalene, acenapthylene, phenanthrene, fluoranthene, benzo[a]anthracene, benzofluoranthenes, benzopyrenes, indenepyrene, benzo[ghi]perylene, and anthanthrene. In contrast, these compounds were not produced in the case of microwave-assisted pyrolysis.     

An open microwave digestion of oxide materials continuously analyzed by ICP emission spectrometry

A device for microwave digestion in an open system was designed, with which a slurry automatically produced from solid sample powders can be processed continuously and then be transported directly into the ICP spectrometer by an online method. The success of this procedure is dependent to a high degree on the behavior of the material to be analyzed and the instrumental device used. In principle, it can be easily installed in a laboratory by combining single units like a mill, a pump, a microwave oven, and an ultrasonic bath in front of an ICP spectrometer. As examples, converter sludge and sinter were processed. In contrast to the sludge, the sinter material was difficult to homogenize sufficiently well. This led to recovery problems, especially for oxides of Si, Al or Ti being separated from the matrix by long-time milling. To overcome the blocking of valves, an open microwave digestion system is applied here (as usual by acid attack). Furthermore, the ICP source is able to excite the particles of a slurry much more efficiently than any other flame. The sufficient results obtained for the online method described here showed it to be a fast method for routine analysis. Received: 16 June 1997 / Revised: 29 July 1998 / Accepted: 31 July 1998     

On-line microwave oven digestion flame atomic absorption analysis of solid samples

A manifold has been developed for on-line microwave oven digestion and flame atomic absorption spectrometric (FAAS) determination of metallic elements in solid samples. The use of a closed flow system permits sample treatment before analysis by FAAS, the direct injection of slurries avoids a filtration step and the interconnection of two conventional rotary injection valves allows the rapid introduction of samples and standards. The determination of lead in sewage sludge was employed as a test system for the proposed on-line sample digestion manifold. The procedure has a limit of detection of 0.2 μg Pb g^?1.     

Electron beam pretreatment of sewage sludge before anaerobic digestion

The pretreatment of waste-activated sludge (WAS) by electron beam irradiation was studied in order to improve anaerobic sludge digestion. The irradiation dose of the electron beam was varied from 0.5 to 10 kGy. Batch and continuous-flow stirred tank reactors (CFSTRs) were operated to evaluate the effect of the electron beam pretreatment on anaerobic sludge digestion. Approximately 30–52% of the total chemical oxygen demand (COD) content of the WAS was solubilized within 24 h after electron beam irradiation. A large quantity of soluble COD, protein, and carbohydrates leached out from cell ruptures caused by the electron beam irradiation. Volatile fatty acids production from the irradiated sludge was approx 90% higher than that of the unirradiated sludge. The degradation of irradiated sewage sludge was described by two distinct first-order decay rates (k ₁ and k ₂). Most initial decay reaction accelerated within 10 d, with an average k ₁ of 0.06/d for sewage sludge irradiated at all dosages. The mean values for the long-term batch first-order decay coefficient (k ₂) were 0.025/d for irradiated sewage sludge and 0.007/d for unirradiated sludge. Volatile solids removal efficiency of the control reactor fed with unirradiated sewage sludge at a hydraulic retention time (HRT) of 20 d was almost the same as that of the CFSTRs fed with irradiated sludge at an HRT of 10 d. Therefore, disintegration of sewage sludge cells using electron beam pretreatment could reduce the reactor solid retention time by half.     

Closed vessel microwave-assisted wet digestion with simultaneous control of pressure and temperature in all vessels

A novel system for microwave-assisted wet digestion in closed vessels is described. Six pressure vessels made of quartz or Hostaflon^? TFM are placed in a special rotor in a microwave oven. During sample decomposition the pressure and temperature are measured in each vessel, and the data are transferred from the rotor by means of infrared light to the control unit. This means that no pneumatic or electronic connections to the vessels need to be established, providing very easy handling. The temperature history of each vessel is recorded; so the progress of each sample digestion may be reconstructed later on. Thus the requirements for quality control in sample digestion are available for the first time. For sample digestion at 75 bar and up to 280° C, quartz vessels with 50 mL volume are used. Depending on the matrix, the maximum sample loading capacity is 1.2 g. TFM-vessels with 100 mL volume work at 30 bar up to 240° C. The performance of the digestion system was tested with twelve standard reference materials. The results closely matched the certified values. Received: 18 September 1997 / Revised: 3 November 1997 / Accepted: 6 November 1997     

Comparison of digestion procedures for the determination of mercury in soils by cold-vapour atomic absorption spectrometry

Five digestion procedures were investigated for the determination of mercury in soils by cold-vapour atomic absorption spectrometry. These methods included three acid leaching procedures in open systems and two acid digestion procedures in closed decomposition vessels. The closed vessels were heated in a conventional laboratory oven or a laboratory microwave oven. In the open systems, digestion with concentrated acids at elevated temperatures led to considerable losses of (organo) mercury compounds, while digestion at ambient temperature gave incomplete oxidation of the sample matrix. To prevent any losses of mercury and to obtain complete oxidation, the use of a closed decomposition system at elevated temperatures appeared to be necessary. The results obtained with these closed systems were in good agreement with those obtained by neutron activation analysis. Heating in a microwave oven appeared to be a considerable improvement over conventional thermal heating.     

微波制样-ICP-AES测定复杂样品中金属元素

分析样品的前处理技术是分析化学中一个非常重要的环节,在样品特别是复杂样品的分析过程中往往是不可缺少的[1],通常需要进行样品预处理后才可上分析仪器进行测定.     

A Comparison of Sample Preparation Procedures for the Determination of Iron and Zinc in Bulgur Wheat by Graphite Furnace Atomic Absorption Spectrometry

ABSTRACT

A comparison was made of different digestion methods for the determination of iron and zinc in two types of bulgur wheat prior to their analysis in graphite furnace atomic absorption spectrometry. Samples were digested by means of acid treatment in room temperature, acid treatment at 60°C in a water bath and in a microwave oven and by dry ashing in open ah system. Except for microwave digestion, the samples proved to be unclear. The concentrations of the elements studied in the bulgur samples obtained by the four digestion methods are inconsistent in limits of standard deviations. It was found that aluminum sulfate (4μg/10μl sample) added to the samples acted as a modifier, improving the signal shape and increasing the stability of slurries obtained from digestion procedures.     

Determination of perfluorooctanesulfonate and perfluorooctanoic acid in sewage sludge samples using liquid chromatography/quadrupole time-of-flight mass spectrometry

A new method is developed for the determination of perfluorooctanesulfonate (PFOS) and perfluorooctanoic acid (PFOA) in sewage sludge samples. The analytes in sewage sludge samples are extracted by methanol and formic acid, cleaned by C18 solid-phase extraction, then separated, identified and quantitated by liquid chromatography/quadrupole time-of-flight mass spectrometry (LC–QTOF-MS). A C18 column (150 mm × 2.1 mm, 3.5 μm) with gradient elution of MeOH–H₂O (60:40) containing 5 mmol/L ammonium acetate and MeOH–H₂O (80:20) is used for the chromatographic separation. [M−K]⁻ ions at m/z 498.93 for PFOS and [M−COOH]⁻ ion at m/z 368.97 for PFOA are selected for QTOF-MS in the negative electrospray ionization mode. The detection limits for PFOS and PFOA in sewage sludge samples are 0.5 and 0.8 ng/g, respectively. The spiked recoveries are in the range of 85–114 and 71–98% for PFOS and PFOA, respectively. The proposed method is successfully applied to the analysis of PFOS and PFOA in 16 sewage sludge samples from China. PFOS and PFOA are detected in most sewage sludge samples and the concentrations of PFOS and PFOA are up to 5383 and 4780 ng/g (oven dry weight), respectively.     

Comparison of Methods for the Preparation of Sewage Sludge Samples Prior to the Spectrophotometric Determination of Phosphorus

Abstract

Three procedures for the preparation of sewage sludge samples prior to the colorimetric determination of phosphorus as “molybdenum blue” were evaluated. Using samples of the US EPA's municipal digested sludge as a reference material, sulfuric acid/ammonium persulfate digestion, muffle furnace ignition followed by extraction of the ash with hydrochloric acid, and direct extraction of the sewage sludge with sodium bicarbonate solution were compared in terms of phosphorus recovery as determined by colorimetric measurements. On the basisof phosphorus recovery, the samples prepared by muffle furnace ignition/hydrochloric acid extraction of the ash showed the best accuracy and precision. This procedure was also superior in terms of the time and effort expended in the preparation of the sewage sludge samples.     

Microwave digestion of sediment, soils and urban particulate matter for trace metal analysis

A microwave digestion technique was developed to determine the content of nine heavy metals in sediments and soils. The digests were subsequently analysed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The metals determined were Ca, Mg, Fe, Mn, Zn, Cr, Cd, Cu, Pb and V. The digestion was achieved by using an acid mixture of HNO₃, HF and distilled H₂O. The experimental study was conducted using four standard reference materials (SRMs): sewage sludge (LGC6136), marine sediment (PACS-1), urban particulate matter (NIST 1648) and coal carbonisation site soil (LGC6138). Two different programs were examined to determine which was optimal for the digestion of real environmental samples. The accuracy and precision of the two digestion programs for the analysis of the SRMs were compared. From the results obtained, the microwave program providing a maximum of power of 650 W and a cycle time of 51 min resulted in the best analytical performance. The experimental results obtained were in good agreement with the certified values and demonstrated that the proposed method is precise and accurate.     

A new procedure for bovine milk digestion in a focused microwave oven: gradual sample addition to pre-heated acid

Milk samples can be efficiently digested using a focused microwave oven, however the conventional procedure of addition of concentrated acids to the liquid sample leads to digestates with elevated acidity and residual carbon concentrations. In this work a focused microwave oven was applied for acid digestion of bovine milk samples using a conventional and an alternative procedure based on gradual sample addition to hot and concentrated acids. A two-level 2³ full factorial design experiment with eight runs was carried out to evaluate the optimum experimental conditions for reducing both the residual carbon and the final acidity of digestates. The three studied parameters were: temperature of the digestion medium for sample addition, addition of sulfuric acid before the sample or during the first step, and number of aliquots of the sample gradually added. The best conditions were attained by adding small aliquots of milk (ten-fold a volume of 0.5 ml added during 5.0 min) to a digestion mixture containing 3.0 ml nitric acid plus 1.0 ml sulfuric acid heated at 105 °C. It was demonstrated that the digestion efficiency of the alternative procedure was better than the conventional procedure, i.e. 98 and 80%, respectively. The alternative procedure was applied for determination of Ba, Ca, Cu, K, Mg, Na, P, and Zn in whole and non-fat bovine milk. The accuracy was proved using two certified reference materials (whole and non-fat milk powder).     

Partial microwave-assisted wet digestion of animal tissue using a baby-bottle sterilizer for analyte determination by inductively coupled plasma optical emission spectrometry

A procedure for partial digestion of bovine tissue is proposed using polytetrafluoroethylene (PTFE) micro-vessels inside a baby-bottle sterilizer under microwave radiation for multi-element determination by inductively coupled plasma optical emission spectrometry (ICP OES). Samples were directly weighed in laboratory-made polytetrafluoroethylene vessels. Nitric acid and hydrogen peroxide were added to the uncovered vessels, which were positioned inside the baby-bottle sterilizer, containing 500 mL of water. The hydrogen peroxide volume was fixed at 100 µL. The system was placed in a domestic microwave oven and partial digestion was carried out for the determination of Ca, Cu, Fe, Mg, Mn and Zn by inductively coupled plasma optical emission spectrometry. The single-vessel approach was used in the entire procedure, to minimize contamination in trace analysis. Better recoveries and lower residual carbon content (RCC) levels were obtained under the conditions established through a 2^4-1 fractional factorial design: 650 W microwave power, 7 min digestion time, 50 µL nitric acid and 50 mg sample mass. The digestion efficiency was ascertained according to the residual carbon content determined by inductively coupled plasma optical emission spectrometry. The accuracy of the proposed procedure was checked against two certified reference materials.     

Comparison of the efficiencies of on-line and high-pressure closed vessel approaches to microwave heated sample decomposition

Two microwave heated digestion techniques have been tested with a view to the evaluation of the effectiveness of wet decomposition procedures: a high-pressure/temperature focused microwave heated and an on-line microwave heated system. The efficiency of decomposition was studied using a number of biological reference materials as well as urine and sewage plant effluent. Oxidation reagents included mixtures of nitric acid, nitric acid/hydrogen peroxide and peroxodisulphate (Oxisolv). The residual carbon content of the digestate was determined coulometrically. Pressurised decomposition of a TFM-Teflon vessel is the most effective procedure (organic material is totally oxidised with nitric acid in a single-step procedure), whereas urine and sewage plant effluent are incompletely decomposed with on-line microwave heated digestion.     

Digestion of plastic materials for the determination of toxic metals with a microwave oven for household use.

A rapid sample-digestion method for the determination of toxic metals, cadmium, chromium, and lead, in polyethylene and polyvinyl chloride has been developed by using a microwave oven for household use. An appropriate amount of the sample taken in a PTFE decomposition vessel was mixed with nitric acid or nitric and sulfuric acids. The vessel was heated in a microwave oven by a predetermined operating program. The digested sample was diluted to a definite volume with water after evaporating most of the nitric acid. The precipitate, if formed, was filtered off by a membrane filter. The metals were determined by ICP-AES. The sample digestion required 5 min (for 20-mg sample) to 25 min (for 60-mg sample). The analytical results obtained for cadmium, chromium, and lead in a polyethylene certified reference material, BCR-680, digested with nitric acid, were in good agreement with the certified values.