M. Soledad Larrechi

Study of the influential factors in the simultaneous photocatalytic degradation process of three textile dyes

The influence of several factors in the simultaneous photocatalytic degradation of three textile dyes - Acid Red 97, Acid Orange 61 and Acid Brown 425 - has been studied using a fractional factorial design 2(5-1). The considered factors were: the initial concentration of each dye, the catalyst concentration (TiO(2)) and pH. First, we developed a rapid analytical methodology based on recording UV-visible spectra during the degradation process and a data treatment using multivariate curve resolution with alternating least squares (MCR-ALS), which enabled the three dyes to be quantified simultaneously despite the overlap of their spectra. The kinetic constant of degradation for each dye in all the experiments was evaluated. In all cases the degradation followed a first order kinetics. For a significance level of 5%, the most important factor in the photodegradation of each dye is the concentration of Acid Red 97, the degradation is more effective at higher pHs and, in the studied range, the concentration of the catalyst is not important.

PARAFAC and MCR-ALS applied to the quantitative monitoring of the photodegradation process of polycyclic aromatic hydrocarbons using three-dimensional excitation emission fluorescent spectra Comparative results with HPLC.

Two methods were developed for the simultaneous quantitative monitoring of photodegradation process of dibenz[a,h]anthracene (DibA), benz[a]anthracene (BaA), benz[a]pyrene (BaP) and benz[k]fluorantene (BkF) using excitation-emission fluorescence spectroscopy. Parallel factor analysis (PARAFAC) and multivariate curve resolution-alternating least squares (MCR-ALS) were satisfactory applied to the data obtained during this process. The results achieved were statistically compared by means of the joint interval test of slope and intercept, with the data obtained using the reference methodology, high performance liquid chromatography (HPLC) method. There are not significant differences between the methodologies proposed and the standard one, and may be a good alternative to the traditional methods of analysis for monitoring the degradation of these pollutants.

Determination of sulphate in water and biodiesel samples by a sequential injection analysis--multivariate curve resolution method.

A spectrophotometric sequential injection analysis (SIA-DAD) method linked to multivariate curve resolution-alternating least squares (MCR-ALS) has been developed for sulphate determination. This method involves the reaction, inside the tubes of the SIA system, of sulphate with barium-dimethylsulphonazo (III) complex, Ba-DMSA (III), displacing Ba(2+) from the complex and forming DMSA (III). When the reaction products reach the detector a data matrix is obtained, which allows a second-order calibration to be developed. The experimental conditions (concentration and sample and reagent volumes) to obtain the highest sensitivity have been chosen applying a 2(4-1) fractional factorial design. The proposed sequential flow procedure permits up to 15 mg SO(4)(2-) L(-1) to be determined with a limit of detection of 1.42 mg L(-1) and it is able to monitor sulphate in samples at a frequency of 15 samples per hour. The method was applied to determine sulphate in natural and residual waters and in biodiesel. The reliability of the method was established for water samples by parallel determination using a standard turbidimetric method for sulphate in natural and residual water samples with results within statistical variation. For biodiesel samples, the method was validated comparing the concentration of some spiked samples with the expected concentration using a test-t.

Sequential injection titration method using second-order signals: Determination of acidity in plant oils and biodiesel samples

A new concept of flow titration is proposed and demonstrated for the determination of total acidity in plant oils and biodiesel. We use sequential injection analysis (SIA) with a diode array spectrophotometric detector linked to chemometric tools such as multivariate curve resolution-alternating least squares (MCR-ALS). This system is based on the evolution of the basic specie of an acid-base indicator, alizarine, when it comes into contact with a sample that contains free fatty acids. The gradual pH change in the reactor coil due to diffusion and reaction phenomenona allows the sequential appearance of both species of the indicator in the detector coil, recording a data matrix for each sample. The SIA-MCR-ALS method helps to reduce the amounts of sample, the reagents and the time consumed. Each determination consumes 0.413ml of sample, 0.250ml of indicator and 3ml of carrier (ethanol) and generates 3.333ml of waste. The frequency of the analysis is high (12 samples h(-1) including all steps, i.e., cleaning, preparing and analysing). The utilized reagents are of common use in the laboratory and it is not necessary to use the reagents of perfect known concentration. The method was applied to determine acidity in plant oil and biodiesel samples. Results obtained by the proposed method compare well with those obtained by the official European Community method that is time consuming and uses large amounts of organic solvents.

UV–visible-DAD and 1H-NMR spectroscopy data fusion for studying the photodegradation process of azo-dyes using MCR-ALS

The photodegradation process of three azo-dyes - Acid Orange 61, Acid Red 97 and Acid Brown 425 - was monitored simultaneously by ultraviolet-visible spectroscopy with diode array detector (UV-vis-DAD) and (1)H-nuclear magnetic resonance ((1)H-NMR). Multivariate curve resolution-alternating least squares (MCR-ALS) was applied to obtain the concentration and spectral profile of the chemical compounds involved in the process. The analysis of the H-NMR data suggests there are more intermediate compounds than those obtained with the UV-vis-DAD data. The fusion of UV-vis-DAD and the (1)H-NMR signal before the multivariate analysis provides better results than when only one of the two detector signals was used. It was concluded that three degradation products were present in the medium when the three azo-dyes had practically degraded. This study is the first application of UV-vis-DAD and (1)H-NMR spectroscopy data fusion in this field and illustrates its potential as a quick method for evaluating the evolution of the azo-dye photodegradation process.

Modelling of the simultaneous photodegradation of Acid Red 97, Acid Orange 61 and Acid Brown 425 using factor screening and response surface strategies.

In this paper the influence of seven variables that could be relevant in the photodegradation of three textile dyes - Acid Red 97, Acid Orange 61 and Acid Brown 425 - has been studied with the aim of determining the most efficient conditions for this process. The type and concentration of catalyst, the presence and concentration of H(2)O(2), the stirring, the pH and the dye concentration have been studied as variables. In the first stage the more basic variables were analyzed using a screening methodology (saturated fractional factorial design) and it was concluded that the most influential variable was the presence of H(2)O(2). In the second stage, a central composite design was used to establish a response surface for the behavior of the photodegradation. In this stage the concentration of Acid Brown 425 was fixed and the degradation was carried out without catalyst. The most remarkable aspects of the experiment are that brown dye is always the most persistent in the solution and that a catalyst is not needed to degrade the dyes quickly. A second-order equation is needed to model this process. The response surface obtained could be useful for reducing the time and money needed to treat effluent wastewater.

MCR-ALS for sequential estimation of FTIR-ATR spectra to resolve a curing process using global phase angle convergence criterion

Curing reactions include side and consecutive reactions while the polymer is growing. In this paper, we used multivariate curve resolution-alternating least squares (MCR-ALS) to obtain quantitative information about the concentration of the chemical species involved in these reactions. The cationic curing reaction between diglycidyl ether of bisphenol A (DGEBA) and gamma-valerolactone (gamma-VL) was monitored by infrared spectroscopy (FTIR-ATR). If the MCR-ALS method is to be used with recorded spectral data, the rank deficiency usually present in the data matrix needs to be overcome and the goodness of the results depends on the initial estimates of the chemical species involved in the reaction. Our strategy was to sequentially apply MCR-ALS in the time intervals where there is selectivity for some reactions and to use the error criterion based on the global phase angle to identify the optimal number of iterations in ALS. The estimated spectra were sequentially incorporated into the data matrix to overcome the rank deficiency. The MCR-ALS results are evaluated by the residuals and parameters such as lack of fit, the percentage of explained variance and the coefficient of dissimilarity between the recovered spectra and the spectra of the pure species, when it is possible. For intermediate species, the correspondence between the ALS spectra solution and the chemical knowledge of this species was also qualitatively evaluated. The goodness of the estimated concentration profile of the two reagents gamma-VL and DGEBA, was evaluated by the correlation coefficient value between the estimated profile and the profile obtained when the specific absorption bands were monitored at 1776 and 910 cm(-1), respectively for each reagent. The correlation coefficient values were 0.9954 and 0.9885, respectively.

Multisyringe chromatography (MSC) using a monolithic column for the determination of sulphonated azo dyes

A methodology based on multisyringe chromatography with a monolithic column was developed to determine three sulphonated azo textile dyes: Acid Yellow 23, Acid Yellow 9 and Acid Red 97. An ion pair reagent was needed because of the low affinity between the monolithic column and the anionic dyes. The proposed analytical system is simple, versatile and low-cost and has great flexibility in manifold configuration. The method was optimized through experimentation based on experimental design methodology. For this purpose two blocks of full factorial 2(3) were done sequentially. In the first experimental plan, the factors studied were: the % of acetonitrile in organic phase, the % of H(2)O in the mobile phase and the kind of ion pair reagent. In this stage, a simple configuration was used which has only one syringe for the mobile phase. After the first experimentation, we added a second syringe with a second mobile phase to the multisyringe module and performed a second full factorial 2(3). The factors studied in this case were: the % of acetonitrile in the second mobile phase, the pH and the concentration of ion pair reagent in both mobile phases. After this design, the optimal conditions were selected for obtaining a good resolution between the peaks of yellow dyes (1.47) and the elution of red dye in less than 8 min. The methodology was validated by spiking different amounts of each dye in real water samples, specifically, tap water, well water and water from a biological wastewater lagoon.

Quantitative analysis of the effect of zidovudine, efavirenz, and ritonavir on insulin aggregation by multivariate curve resolution alternating least squares of infrared spectra

Quantification of the effect of antiretroviral drugs on the insulin aggregation process is an important area of research due to the serious metabolic diseases observed in AIDS patients after prolonged treatment with these drugs. In this work, multivariate curve resolution alternating least squares (MCR-ALS) was applied to infrared monitoring of the insulin aggregation process in the presence of three antiretroviral drugs to quantify their effect. To evidence concentration dependence in this process, mixtures at two different insulin:drug molar ratios were used. The interaction between insulin and each drug was analysed by (1)H NMR spectroscopy. In all cases, the aggregation process was monitored during 45 min by infrared spectroscopy. The aggregates were further characterised by scanning electron microscopy (SEM). MCR-ALS provided the spectral and concentration profiles of the different insulin-drug conformations that are involved in the process. Their feasible band boundaries were calculated using the MCR-BANDS methodology. The kinetic profiles describe the aggregation pathway and the spectral profiles characterise the conformations involved. The retrieved results show that each of the three drugs modifies insulin conformation in a different way, promoting the formation of aggregates. Ritonavir shows the strongest promotion of aggregation, followed by efavirenz and zidovudine. In the studied concentration range, concentration dependence was only observed for zidovudine, with shorter aggregation time obtained as the amount of zidovudine increased. This factor also affected the aggregation pathway.

Kinetic analysis of C.I. Acid Yellow 9 photooxidative decolorization by UV-visible and chemometrics.

A kinetic study of the C.I. Acid Yellow 9 photooxidative decolorization process, using H(2)O(2) as oxidant, was carried out by chemometric analysis of the UV-visible data recorded during the process. The number of chemical species involved in the photooxidative decolorization process was established by singular value decomposition (SVD) and evolving factor analysis (EFA). Information about the different chemical species along the process was obtained from the spectral and concentration profiles recovered by soft multivariate curve resolution with alternating least squares (MCR-ALS). This information was complemented by mass spectrometry (MS) to postulate a reaction pathway. The dye photooxidative decolorization process involved consecutive and parallel reactions. The consecutive pathway consists of a first stage of dye oxidation followed by the rupture of the azo linkage to form smaller molecules that are degraded in a subsequent stage. The parallel reactions form products that are undetectable in the UV-visible spectra. Kinetic constants of the reactions postulated in the photooxidative process were retrieved by applying a hybrid hard and soft MCR-ALS resolution. All constants were similar for the consecutive stages and higher than those obtained for the parallel reactions.