FIA-coupled spectrophotometric method for

Environ Monit Assess (2018) 190:617 https://doi.org/10.1007/s10661-018-6984-9

FIA-coupled spectrophotometric method for determination of Cr (VI) traces in natural waters: application of in-line dissolution of 1,5-diphenylcarbazide after heat treatment and activated alumina as adsorbent for preconcentration Diogenes Meneses & José Guimarães F. Júnior & Paulo Cesar Costa de Oliveira Received: 23 April 2018 / Accepted: 18 September 2018 # Springer Nature Switzerland AG 2018

Abstract This work proposes the quantification of Cr (VI) ions in natural waters in trace level, using activated alumina (Al2O3) as preconcentration support, controlled in-line dissolution of the solidified chromophore diphenylcarbazide after heat treatment and spectrophotometric detection. The manifold ensures high sensitivity of analytical response, good repeatability, and stability. In this work, optimization of experimental conditions of a flow injection system was chosen as the parameters for greater sensitivity and better selectivity. The selected optimized conditions were 0.30 mol L−1 for H 2 SO 4 concentration, system flow rate as 0.40 mL min − 1 , sample injection volume of 192.50 μL, 2 min for preconcentration time, and 0.10 mol L−1 for eluent concentration. The analytical curves obtained for real sample analysis show linear range from 0.192 to 0.961 μM, linear correlation coefficient R = 0.9997 and LOD = 0.024 μM. The preconcentration factor of about four times was obtained through the passage of 800 μL of a standard solution containing 0.961 μM of Cr (VI) through mini-column of preconcentration followed by elution at 192.5 μL of NH4OH 0.1 mol L−1 solution. The solid chromogenic reagent presented high durability (weeks in daily use with mass of 0.0993 g) and good reproducibility in analytical signal. The reactivation of the mini-column of alumina should be executed after ten injections of D. Meneses (*) : J. G. F. Júnior : P. C. C. de Oliveira Instituto de Química e Biotecnologia, Universidade Federal de Alagoas, Maceió, AL, Brazil e-mail: [email protected]

eluent, using 800 μL of HCl 0.02 mol L−1 solution in flow through the same. Each cycle of injection and elution of the sample takes about 5 min on the proposed terms. Despite the length of each cycle still be high, low concentrations can be detected using a technique of relatively low cost. This is due in part, the association dissolution of the chromogenic reagent directly in the line and the preconcentration step. Another important factor is the economy of reagent chromogenic, low generation of reject contributing to better quality of the environment, and the high potential for applications to work in field. Keywords Chromium determination, 1,5Diphenylcarbazide treatment . Flow injection analysis . Spectrometry . Water samples

Introduction Flow injection analysis systems (FIA), proposed in 1975 by Jaromir Ruzicka (Plutschack et al. 2017; Ṙuzicka and Hansen 1975; Trojanowicz and Kołacińska 2016) have been used for the mechanization and automation of chemical analyses. These systems are quite attractive because it is possible to apply important chemical analysis process steps such as separation, preconcentration, addition of reagents in line as well as great potential for determination of physical chemical parameters, such as diffusion coefficients, complex binder capacity, kinetic parameters, and reaction

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stoichiometry (Abouhiat et al. 2017; Elsuccary and Salem 2015; Kucza 2013; Palamy and Ruengsitagoon 2017). Chromium is an essential metal for the growth and metabolism of living organisms, but it can be extremely harmful depending on its oxidation state (Jaishankar et al. 2014). For example, Cr(III) is considered a micronutrient and has diverse effects on the human metabolism (Hernandez et al. 2018). On the other hand, Cr(VI) is considered a contaminant in water sources and a carcinogenic metal causing several diseases although the mechanism of the carcinogenic process is yet not clear (Bernadette et al. 2015; Chiu et al. 2010; Vaz et al. 2017). The official method for Cr(VI) determination in water is based on the spectrophotometric use of 1,5diphenylcarbazide (DPC) as chromophore (Taylor 1997). The reaction between DPC and Cr(VI) in acid medium yields a chromogenic product of intense violet color, monitored at 540 nm. However, the use of DPC for this purpose presents problems such as low solubility in water and instability, which hinders the use of its solution for a long period of time and thus requires daily preparations. To overcome these difficulties, propanone or ethanol is often used to increase the solubility of the DPC, but despite satisfactorily resolve the solubility problem, these solutions are even more unstable. To minimize the instability of some reagent solutions, inline dissolution through the application of flow injection analysis systems has been proposed as a promising alternative. Haj-Hussein and coworkers (Haj-Hussein et al. 1986) proposed the use of solid reagent with inline dissolution, where a chromatographic column packed with cupric sulfide was developed to allow the determination of cyanide ion by FIA/AAS, which obtained a relevant improvement in the method sensitivity and precision. Andrade and coworkers (Andrade et al. 1996) published the first work using DPC as a solid chromophore for spectrophotometric determination of Cr(VI) by FIA. The alternative procedure involved the in-line dissolution of this reagent, a homogeneous mixture of the solid DPC together with silica (25% w/w) packed into a PTFE column, which showed a great efficiency due to decrease of dispersion value and low LOD due to absence of peaks of index of refraction. The determination of Cr(VI) traces has attracted a great attention because of environmental and biological concerns (Yang et al. 2015). Since the Cr(VI) concentration found in water is generally below 1 μg L−1,

Environ Monit Assess (2018) 190:617

reliable detection requires good sensitivity from the technique employed. Several preconcentration procedures for the determination of Cr(VI) have been developed in order to improve sensitivity of analytical methods (Alves and Coelho 2013; Boutorabi et al. 2017; Çimen et al. 2013; Ozdemir et al. 2017). The feasibility of preconcentration of Cr(VI) present in natural waters to the activated alumina using FIA with spectrophotometric detection was investigated by Pannain and Santelli (Pannain and Santelli 1995). The chemical and flow variables and the influence of the interfering species were studied with and without preconcentration systems. Sperling and coworkers (Sperling et al.’ 1992) reported a rapid and sensitive method for selective determination of Cr (III) and Cr(VI) species in water samples by flame atomic absorption spectrometry using online preconcentration in a micro-column packed with alumina activated in the acid form. It should be emphasized that when preconcentration procedures are applied, the increase in the time of analysis is inevitable. This can be overcome by the proper choice of the adsorbent and bypassing the preparation of the chromophore solution using the solidified and packed in a column reagent. The use activated alumina as adsorbent shows excellent selectivity for chromium species (Mahmoud et al. 2008; Mor et al. 2007) when submitted to the Clark and Lubs buffer, together with low cost, ease of manipulation for activation and easy recovery for reuse. Therefore, the aim of this work is to develop an analytical methodology for the determination of Cr(VI) in natural water samples at trace level using controlled in-line solid reagent dissolution (DPC) and preconcentration in activated alumina mini-column, associated to FIA with spectrophotometric detection.

Experimental Reagents and solutions All chemicals used were analytical reagent grade with no further purification, purchased from Sigma-Aldrich (Sigma, St. Louis, MO, USA). Milli-Q ultrapure water, 18 MΩ cm (Millipore, Kansas City, MO, USA) was used for making solutions. Stock solution of Cr(VI) (9.6 mM) was prepared dissolving K2Cr2O7 in water; All solutions were stored in amber flask at room temperature, except the NH4OH solution which was stored


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in a polyethylene flask. All working solutions were prepared by appropriated dilution with H2O.

a relatively uniform appearance which makes it suitable for on-site analysis.

Instrumentation

Preparation of the mini-column containing the activated alumina

The research was carried out in a 700 Plus FEMTO spectrophotometer (FEMTO, Brazil) with a U-flow cell with 10 mm optical path, 310 μL volume and polyethylene pumping tubes with 0.76 mm of internal diameter; an Ismatec Reglo CP78016-30 8-roller peristaltic pump (IDEX Health & Science SA, Wertheim, Germany) was used to pump the solutions to the detector. An IKA heating plate model RH basic KT/C was used for the preparation of the DPC after heat treatment process; Eppendorf pipette tip with 1000 μL capacity were used to house the solid reagent billet creating a mini-column containing the solid DPC; drying and sterilization oven model 315 SE was employed for the drying and activation of alumina; pHmeter Mars Mark 2000 was used for pH adjustments. Preparation of the mini-column containing DPC after heat treatment process Based on the methodology of El-Kabbany and coworkers (El-Kabbany et al. 2011), the DPC was prepared by weighing 0.7 g of the solid reagent in a 50-mL beaker followed by heat treatment at 173 °C and the liquid reagent poured into a mold made of aluminum foil. After cooling at room temperature, the mold was removed and the solidified reagent was weighed to obtain 0.5444 g which was carefully inserted into a 1000-μL Eppendorf pipette tip. Glass wool was positioned at the end of the tip, to avoid possible particle release from the solidified reagent were carried to the system at the time of in-line flow dissolution. Figure 1 shows that the reagent billet is feasible, portable and has

Fig. 1 Solidified DPC cylinder

Firstly, 10.00 g of alumina was treated with HCl (50 mL, 2 mol L−1) to leach impurities and rested for 24 h before HCl solution was removed. The alumina was then washed with distilled water in order to remove all excess hydrochloric acid through the test for chloride in the silver nitrate wash waters. After this step, the alumina was placed in an oven for drying under a temperature of 80 °C for 24 h to activate its sites. Then, the alumina was transferred to a desiccator and reweighed after cooling. 9.25 g of silica was obtained, indicating a loss of only 7.50% of the initial mass of alumina. The activated alumina mini-column was prepared by the introduction of 90 mg in a cylindrical polyethylene tube (16 mm in length and 3.5 mm in internal diameter), allowing little reagent consumption and easy transportation. Glass wool was positioned at the end of the column to prevent possible alumina particles from entering the system and interfering the analytical signal. FIA system for preconcentration and determination of Cr(VI) trace in water The optimized experimental procedure for Cr(VI) preconcentration and determination in natural waters was carried out as described: 0.76 mm internal diameter pumping tubes, 0.40 mL min−1 system flow, eluent (NH4OH 0.10 mol L−1) with in-line injection of 192.50 μL, carrier solution (H2SO4 0.05 mol L−1), Clarck-Lubs buffer solution pH 2 for activated alumina conditioning flow for 2 min, 2 min Cr(VI) preconcentration time and spectroscopic measurements at λ = 548 nm. The standard procedure was carried out according to the following steps: after FIA system assembling, washing was carried out with ultrapure water for 10 min; Both DPC dissolution and carrier solutions were streamed until stability of the analytical signal; Clarck-Lubs pH 2 buffer solution in-line and flow for 2 min by the activated alumina mini-column to condition it in acid medium and to make it selective to adsorb Cr(VI); Cr(VI) was preconcentrated in the activated alumina in continuous flow for 2 min; adsorbed analyte in the activated alumina was eluted by flow injection of 192.50 μL 0.10 mol L−1 NH4OH; the

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solution from the DPC after heat treatment process, which underwent in-line dissolution by 0.05 mol L−1 sulfuric acid, with the Cr(VI) eluted from the alumina minicolumn activated for the subsequent UV-Vis at λ = 548 nm and then proceed to discard; after every 10 cycles of preconcentration, the silica was reactivated with 0.02 mol L−1 hydrochloric acid in continuous flow for 5 min. The flow injection analysis module developed in this work for the application of the proposed method is illustrated in Fig. 2.


showing higher sensitivity for the determination of Cr(VI) in a pH range between 0 and 1. Thus, the H2SO4 concentration was studied as DPC carrier and dissolution solution. As shown in Fig. 3a, increasing the concentration of H2SO4 leads to greater DPC dissolution by increasing the response of the analytical signal. However, loss in analytical signal due to DPC excess in the reaction using concentrations above 0.40 mol L−1 is observed; Thus, the selected H2SO4 concentration for DPC stability study was 0.30 mol L−1 due to suitable signal response. System flow rate

Results and discussion Optimization of the analytical system Concentration of the carrier solution In order to increase the sensitivity of the proposed method, the influence of acid concentration used as the carrier solution was studied since its concentration has a direct influence on the dissolution of the solid reagent. The absorbance of the DPC solution is pH-dependent,

The solution flow rates study was carried out to observe the analytical signal behavior and its stability. Thus, a pumping tube was analyzed with different the peristaltic pump output pressures. Fig. 3b shows that as the solution flow rate increases, there is an exponential drop in the analytic signal. Therefore, the residence time or contact of carrier solution for in-line dissolution of the chromogenic reagent affects the analytical response and it is inversely proportional to the flow rate of carrier solution in the system. Consequently, the lower the flow the longer

Fig. 2 a Scheme of the analysis module for the stability study of solidified reagent analytical signal. b Scheme of the analysis module for preconcentration and determination of Cr (VI) in water samples


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Fig. 3 a Carrier solution concentration. b System flow rate. c Sample injection volume. d Reaction coil length

the contact time of the carrier solution in the DCP column, increasing the concentration of the dissolved chromogenic reagent with superior analytical signal. Therefore, the flow rate chosen for the proposed methodology was 0.40 mL min−1, as it presented a superior analytical signal, as well as low reagent consumption.

Sample injection volume In order to optimize this step, the volume of injected sample and the variation of the analytical signal were studied. In Fig. 3c, it is observed that the analytical signal increases proportionally with the volume of

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sample injected. However, this increase has a maximum limit, tending to be constant due to the excess of sample in relation to the chromogenic reagent in the system and/ or to the time necessary for the reaction between sample and chromogenic reagent to occur. So, the volume of 192.50 μL of sample injected for use in the determination of the analyte was chosen because it shows good relationship between sample volume and analytical signal, as well as small production of chemical residues. Reaction coil length Reaction coil in FIA systems is employed to assist the interaction between the segments of reagent solution in order to increase the mixing efficiency between them (Dias et al. 2006; Vishnikin et al. 2012). Thus, the study of the length of the reaction coil was carried out in this work and it was observed an improvement in the analytical response as the length of the reaction coil increased. However, it was observed that large reaction coil lengths generate excessive response time and, therefore, increased time analysis and residue production and decreased analytical signal sensitivity due to the dispersion of sample segments and reagents in the system. In Fig. 3d, it is observed that the use of the reaction coil provided an increase of the analytical signal, however, for reaction coils greater than 100 cm the phenomenon of dispersion becomes the major process causing analytical signal decrease. Therefore, in order to carry out trace analysis, it is not convenient to use a reaction coil since increases the reaction time of the reaction segment in the system. So, the reaction coil was not employed in the proposed methodology. Optimization of the system developed for Cr(VI) preconcentration and determination Efficiency of the alumina mini-column adsorption The study of Cr(VI) adsorption efficiency in the alumina mini-column is essential because it is responsible for the indication of the amount of analyte that will be actually adsorbed. Thus, the following steps were performed: (1) conditioning of the alumina mini-column with the Clarck-Lubs buffer (pH 2) in-line flow with flow rate of 0.40 mL min−1 for 5 min; (2) preconcentration of Cr(VI) 180.0 μg L−1 for 15 min and, alongside with preconcentration, the waste was collected in a cleaned container; (3) the waste container was moved to avoid


contamination with the eluent since it was employed in the fifth step; (4) to ensure the effective elution of Cr(VI) adsorbed, injection of 192.50 μL of 0.10 mol L−1 NH4OH, which showed absorbance values of 0.329, 0.012 and 0.000 in the first, second, and third injection, respectively; (5) repeated the fourth step with waste from the second step, obtaining for the three injections, consecutively, an absorbance signal equal to 0.001 arbitrary unit; (6) several preconcentration procedures with elutions were carried out after conditioning the alumina to observe the alumina efficiency in Cr(VI) adsorption and it was verified that after ten samples the alumina decreases its efficiency considerably. Thus, it was found that the alumina mini-column has an excellent efficiency when conditioned with the Clarck-Lubs buffer at pH 2, for Cr(VI) adsorption; however, after ten cycles, the alumina must be reconditioned with HCl 0.20 mol L−1 in continuous flow at a flow rate of 0.40 mL min−1 for 5 min. Preconcentration time The preconcentration time is directly related to the reagent consumption, time feasibility and the sensitivity of the analytical signal, and for the latter, it is understood that the higher the preconcentration time the greater the analytical signal (Ohshima et al. 2014). Figure 4a, b show that as the preconcentration time increases, the amount of preconcentrated Cr(VI) in the mini alumina column is expected to increase. Thus, the preconcentration time of 2 min was chosen, due to less production of residues. Sample loading in the alumina mini-column for preconcentration This study is linked to Cr(VI) adsorption kinetics in the alumina mini-column. As the flow of the system is related to the volume that passes through a given point and in a time interval, it can be affirmed that the flow is related to the residence time of Cr(VI) in the minicolumn of alumina. Figure 4c shows that by increasing the flow rate there is a considerable decrease in the analytical signal. This can be explained by the residence time of the analyte in the adsorption mini-column since lower residence time of the analyte leads to its lower adsorption. This study demonstrated that for flow rates greater than 1.20 mL min−1 the increase of internal pressure in the pipes leads to leakage of the system.


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Fig. 4 a Preconcentration time with 0.1 M NH4OH. b Preconcentration time with 2 M NH4OH. c System flow rate with alumina minicolumn. d Eluent concentration. e Carrier solution concentration

Thus, the system flow that presented the best conditions for the proposed method was 0.40 mL min−1 because it shows better time and analytical signal correlation. Concentration of the eluent and its influence on the analytical signal This study was carried out to evaluate the influence of the concentration of the eluent on the analytical signal and its interaction with the solidified reagent solution, so Cr(VI) preconcentration was performed with ultrapure water instead of the alumina mini-column, making a blank of the system. Figure 4d shows that high concentration of the eluent (NH4OH) causes relevant interference, and may mask the results. This is possible related to the rapid and spontaneous oxidation of 1,5diphenylcarbazide to diphenylcarbazone in alkaline

medium (Tunçeli and Turker 2002). Thus, to overcome this problem, the eluent should be at a low concentration, so the concentration chosen for the eluent was 0.10 mol L−1 because it presented better performance in relation to the analytical signal and because the limiting concentration allowed to guarantee the total elution of the analyte. Concentration of the carrier solution and its influence on the dissolution of the chromogen and on the analytical signal A relevant problem found for the spectrophotometric analysis system optimization is the Schlieren effect (Dias et al. 2006; Vidigal and Rangel 2015; Vishnikin et al. 2012) which is the formation of intense concentration gradients in the FIA system which causes

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refractive index gradients in the sample zone. Another factor that may influence the magnitude of the Schlieren effect is the mixing of the carrier solution and the addition of reactants by confluence. In this work, besides the confluence used for the transporter and the analyte, in-line dissolution of the chromogen was added to avoid DPC instability. Thus, analytical signal of the interaction of Cr(VI) and DPC at high concentrations produces the Schlieren effect. To overcome this problem, the concentration of the carrier solution, H2SO4, was studied since it directly influences the in-line dissolution of the DPC. Therefore, it was found that increasing the concentration of H2SO4 the DPC dissolution also increased, as expected. Therefore, for the proposed methodology, the concentration of the carrier solution should not be greater than 0.05 mol L−1. However, higher concentrations of H2SO4 should be used to determine Cr(VI) at concentrations above 0.96 μmol L−1. As can be seen in Fig. 4e, the analytical signal started with negative signal and increased until it became a positive signal for the same injection. This factor confirmed that the concentration of the carrier solution causes the Schlieren effect if it is at a concentration higher than 0.05 mol L−1 in the experimental conditions proposed for the preconcentration and spectrophotometric determination of Cr(VI) ions in natural waters. So, the concentration for the carrier solution used in this work was 0.05 mol L−1. Interference study The interference study was carried out to evaluate the effect caused by interfering ions in the concentration range of 9.62 μmol L −1 to 7.69 mmol L −1 to a 0.96 μmol L−1 Cr(VI) solution. Blank solutions were measured at the beginning and at the end of analysis and the analytical signal of a standard solution containing Cr(VI) 0.96 μmol L−1 compared with the signals from the possible interferers. Fe(II), Cu(II), Zn(II), Fe(III), Ni(II), and Mn(II) were evaluated as interfering species due to their incidence in natural waters. A great advantage of the system configuration proposed in this work is the in-line preconcentration step as well as the in-line dissolution of the chromogenic reagent which joins the analyte only after the elution step. Thus, if alumina does not have adsorptive affinity with the interfering species, the analytical signal remains the same. However, if the interfering specie has an adsorptive affinity with the alumina the binding sites that would only have to be

occupied by Cr(VI), the analytical signal after the elution will decrease because of partial Cr(VI) preconcentration (Boutorabi et al. 2017; Soares et al. 2009). Besides, the reduction of Cr(VI) to Cr (III) due to the interaction with interference ions such as Fe(II) or competition with analyte for DPC binding sites also leads to decrease in the analytical signal (Buerge and Hug 1997; Vaz et al. 2017). It was noticed, however, that if alumina is appropriately conditioned its selectivity towards Cr(VI) is increased, reducing interference. Although alumina shows great selectivity for Cr(VI) determination some species may interfere considerably and, according to Table 1, this interference is only noticeable when concentration is at least 10-fold for Fe(II), 20-fold for Mn(II), and 100-fold for Ni(II), Fe(III), and CO32−.

Alumina preconcentration factor and figures of merit In order to evaluate the preconcentration procedure it is necessary to correlate the amount of Cr(VI) adsorbed on the alumina with the initial Cr(VI) concentration in the standard. This way, a study was carried out using the following steps: (1) from the system flow and the preconcentration time the volume of the standard with predefined concentration was calculated; (2) the theoretical Cr(VI) mass was calculated by the initial concentration of the standard and the volume used in the preconcentration that was retained in the mini alumina column; (3) volume was recorded after optimizing the eluent volume for total Cr(VI) removal from the mini alumina column; (4) the concentration of Cr(VI) eluted

Table 1 Effect of interfering species on the analytical signal of Cr (VI) Evaluated species

Tolerance (μM)

Cu(II)

6290

NO3−

6450

Zn2+

6120

2.74

Cl−

8800

3.62

Fe2+ 3+

1.34

Error (%) 1.39 1,71

3.17

Fe

89.5

Mn2+

18.2

3.06

Ni2+

85.2

18.00

CO32−

167

7.14

3.13


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Table 2 Determination of Cr(VI) by different methodologies and the proposed work Detection technique

Limit of detection

Work range

Reference

Laser-induced fluorescence

0.094 nM

0–100 nM

(Peng et al. 2017)

Molecular fluorescence

5 nM

0.01–250 μM

(Zhang et al. 2018) (Lakshmi Narayana et al. 2010)

Molecular spectroscopy

0.09 nM

9.61–148.07 μM

Molecular fluorescence

40 nM

0–140 nM

(Cai et al. 2014)

Molecular spectroscopy

0.024 μM

0.192–0.961 μM

This work

was calculated from the mass retained in the alumina mini-column, calculated in the second step, and the ideal eluent volume for the total removed from the analyte of the alumina mini-column described in the third step. (5) Last, the ratio between the new concentration found and the initial concentration of the analyte of interest was calculated. This ratio is called the preconcentration factor. The preconcentration factor was established for a standard time and concentration. This way, for a 76.93 nmol L −1 Cr(VI) solution and 20 min of preconcentration time, preconcentration factor was set as 83. Thus, alumina is an excellent adsorbent for Cr(VI) species, being applied for the proposed method. The linearity of the analytical signal as a function of the Cr(VI) concentration using the controlled and in-line dissolution of DPC after thermal treatment with spectrophotometric detection was verified through the analytical curve. Thus, three calibration curves with

Table 3 Recovery assay for the proposed work

Water sample

different concentration ranges for Cr(VI) and different preconcentration and buffering times were used in order to evaluate the repeatability, reproducibility, and sensitivity of the proposed methodology as well as preconcentration factor. It was also verified that the sensitivity of this methodology is intrinsically related to the preconcentration time, and we can observe this by the limit of detection (LOD), which presented the following values: 3.45 nmol L−1 for a concentration range of 19.23 to 134.63 nmol L −1, buffering time of 5 min and preconcentration time of 20 min, R = 0.9998; 41.34 nmol L−1 for a concentration range of 96.16 to 480.81 nmol L −1 , buffering time of 5 min and preconcentration time of 5 min, R = 0.9969; and 30.68 nmol L−1 for a concentration range of 769.29 to 1.538.58 nmol L −1 , buffer time of 2 min and preconcentration time of 2 min, R = 0.9996, in the latter, buffering times were decreased to ensure that part of the analyte was not discarded due to adsorption.

Type

Cr(VI) (nM) Spiked

1

2

3

Waste water

Fresh water

Tap water

Recovery (%) Recovered < LOD

–

192.3

201.0

104.5

576.9

572.5

99.2

961.5

976.8

101.6

0.0

< LOD

–

192.3

201.0

104.5

576.9

557.3

96.6

961.5

944.1

98.2

0.0

< LOD

–

192.3

208.5

102.1

576.9

559.6

97.0

961.5

936.2

97.1

0.0

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An excellent correlation was achieved even at low concentrations of Cr(VI), with linear correlation coefficient R = 0.9998. Moreover, the proposed methodology shows good stability, repeatability, and sensitivity with limit of detection (LOD) of 12 nmol L−1. These data demonstrate the proposed methodology is an excellent and powerful tool for chromium analysis at trace level. In order to evaluate the degree of sensitivity of the proposed method, a comparison of detection limits with different analytical techniques for Cr(VI) determination was made. Table 2 shows that the proposed method presents excellent sensitivity due to good LOD compared to other techniques and methodologies.

Thus, through the analysis of the results of the real sample, this developed methodology presented an excellent and powerful tool to be applied for analysis of Cr(VI) trace with low cost and excellent performance proven by LOD found and compared with other techniques and methodologies which present greater costs in the analysis and maintenance of equipment. This methodology can be applied to preconcentration of other chemical species, which is the object of future study.

Real sample analysis

References

A recovery assay for the proposed methodology for Cr(VI) preconcentration and determination was employed in the analysis of three water samples (waste, fresh, and river). Water sampling and preservation were performed as described elsewhere (Soares et al. 2009). The recovery assay showed recovery in the range of 96.6 to 104.5% of Cr(VI) showing that the method is suitable for determination of Cr(VI) trace in water samples. These results can be seen in Table 3.

Conclusions The proposed analytical methodology developed shows excellent suitability, sensitivity, and selectivity for the preconcentration and Cr(VI) determination at trace level in waters, and this can be ratified by the results presented in this work. The in-line controlled dissolution of the DPC chromogenic reagent after heat treatment, one of the main focus of this work, was extremely relevant and effective in solving problems related to the instability of DPC solutions, the daily preparation of this solution and proving to be an excellent tool to be used in routine analyses. This contributes significantly to green chemistry by not generating alcohol and ketone-based waste, as well as generating less DPC-based waste. The work also confirmed that, once activated and conditioned according to the data presented in this work, alumina is suitable for Cr(VI) adsorption. Besides, it can be applied for depollution of water bodies contaminated with Cr(VI) associated with the in-line dissolution of DPC and shows great potential for in situ quantification of Cr(VI) in natural waters, even at low concentrations.

Acknowledgements The authors thank CAPES, CNPq, and FAPEAL for financial support and scholarships.

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