Abstract
Monitoring of toxic trace metals such as arsenic, cadmium, chromium and lead in water samples are of a great significance from the health risk point of view. This is because of the increased concentrations of these metals in water systems as the result of industrial and human activities. Therefore, there is a need for development of accurate methods for their determination and consequent removal. Hence the aim of this study was to prepare adsorbents for extraction, preconcentration and removal of arsenic, cadmium, chromium and lead in various water systems. Thus, MoS42--intercalated magnetic FeMgAl layered double hydroxide (Fe3O4@MoS42--FeMgAl LDH) nanocomposite and NiFe-LDH@AC composite were prepared and applied as adsorbernts for analysis and removal of As, Cr, Cd and Pb in water samples. These materials were characterized using scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy (SEM-EDS), Zeta potential, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) spectroscopy and Brunauer–Emmett–Teller (BET). These adsorbents were applied for development of two sample preparation methods, that is, magnetic solid phase extraction (MSPE) and ultrasound assisted dispersive solid phase extraction (UA-DSPE) in water samples prior to their determination by inductively coupled plasma optical emission spectroscopy (ICP-OES). The parameters affecting the experimental procedures were optimised using fractional factorial design and reseponse surface methodology.
Under the optimized conditions, the MSPE/ICP-OES and UA-DSPE/ICP-OES methods were validated in terms of limit of detection (LOD), limit of quantification (LOQ), accuracy, linearity and precision. When using MSPE/ICP-OES the LOD and LOQ for Cd were 0.03 μg/L and 0.1 μg/L, whereas the LOD and LOQ for Pb were 0.056 μg/L and 0.18 μg/L, respectively. The calibration curves showed good linear range from 0.1–800 μg/L and 0.2-750 μg/L with coefficient of determinations (R2) of 0.9989 and 0.9975 for Cd and Pb, respectively. The MSPE/ICP-OES method was applied for determination of Cd and Pb using certified reference materials and spiked samples. The method displayed acceptable accuracy and precision, expressed in terms of %RSD of less than 5. The resultant recoveries were ranging from 98.7-99.4%, respectively. Method application was further validated by analysing cadmium and lead at trace levels in river water, tap water and wastewater samples. The concetration of Cd and Pb ranged from 0-1.77 μg/L and 0-6.53 μg/L, respectively.
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The analytical performance of UA-DSPE coupled with ICP-OES based on NiFe-LDH@AC composite was expressed in terms of linearity, LOD, precision and accuracy for As and Cr. Linearity was investigated at 0.1-150 μg/L and 0.07-100 μg/L and LOD, LOQ were reported as (0.021-0.031 μg/L) and (0.1-0.07 μg/L), respectively. Precision expressed in terms of %RSD was less than 5 and accuracy was reported as 92.6-99.2%, respectively. In addition, adsorption isotherm and adsorption kinetic models were used to investigate the the adsorption mechanisms. The adsorbent displayed high adsorption capacities of 102 mg/g and 92.6 mg/g for As and Cr, respectively. The concentrations of As and Cr in wastewater samples ranged from 0-7.89 μg/L and 0-33.7 μg/L, respectively. Finally, the NiFe-LDH@AC composite was explored for the removal of target analytes and the removal efficiency for spiked wastewater samples ranged from 89.7-100%.