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The study aims at synthesizing a simple, cost-effective, selective, and sensitive colorimetric sensor based on chlorophyll-coated silver nanoparticles (Chl-AgNPs) using microwave-assisted method for the determination of Cr(VI) ions in an aqueous medium. The characterization reveals that synthesized nanoparticles are 20–30 nm in size and spherical in shape. The presence of methyl, carboxyl, aldehyde, and ketone groups on the surface of Chl-AgNPs results in the formation of size controlled, well dispersed, and stable silver nanoparticles. The prepared nano-colloidal solution has been used for selective and sensitive determination of Cr(VI) in water samples. The addition of single drop of Chl-AgNPs solution immediately changes the color of water samples containing Cr(VI) to red displaying good sensitivity from 10 to 100 µM and high selectivity even in the presence of other metal ions. The ultraviolet–visible spectra show that in the presence of Cr(VI), the localized surface plasmon resonance (LSPR) band of Chl-AgNPs at 410 nm shifts to 470 nm with a new peak at 357 nm. Furthermore, the absorbance of the LSPR band decreases and the absorbance at 357 nm increases linearly with the concentration of Cr(VI). The functional groups on the surface of nanoparticles bind Cr(VI) on the surface leading to agglomeration and alterations in interparticle distance that are observed as changes in the color and LSPR profile. The limits of detection and quantification for Cr(VI) ions were determined to be 0.62 µM and 1.88 µM, respectively. [ABSTRACT FROM AUTHOR]

We combined a nickel–aluminum–zirconium complex hydroxide (NAZ) with colloidal silica as a binder to prepare a granulated agent for adsorbing heavy metals from aqueous media. Three samples with different particle diameters were prepared to evaluate the effects on the properties: small (NAZ-S), medium (NAZ-M), and large (NAZ-L). We confirmed the granulation of the prepared samples at a binder content of 25%. NAZ-S had the largest specific surface area and number of hydroxyl groups, followed by NAZ-M and then NAZ-L. Regarding the adsorption capacity, NAZ-S adsorbed the most chromium(VI) ions followed by NAZ-M and then NAZ-L. The binding energy of Cr(2p) at 575–577 eV was detected after adsorption, and the effects of the temperature, contact time, and pH on the adsorption of chromium(VI) ions were evaluated. We identified the following adsorption mechanism: ion exchange with sulfate ions in the interlayer region of the NAZ samples. Finally, the chromium(VI) ions adsorbed by the NAZ samples were easily desorbed using a desorption solution. The results showed that NAZ offers great potential for the removal of chromium(VI) ions from aqueous solutions. [ABSTRACT FROM AUTHOR]

We combined a nickel–aluminum–zirconium complex hydroxide (NAZ) with colloidal silica as a binder to prepare a granulated agent for adsorbing heavy metals from aqueous media. Three samples with different particle diameters were prepared to evaluate the effects on the properties: small (NAZ-S), medium (NAZ-M), and large (NAZ-L). We confirmed the granulation of the prepared samples at a binder content of 25%. NAZ-S had the largest specific surface area and number of hydroxyl groups, followed by NAZ-M and then NAZ-L. Regarding the adsorption capacity, NAZ-S adsorbed the most chromium(VI) ions followed by NAZ-M and then NAZ-L. The binding energy of Cr(2p) at 575–577 eV was detected after adsorption, and the effects of the temperature, contact time, and pH on the adsorption of chromium(VI) ions were evaluated. We identified the following adsorption mechanism: ion exchange with sulfate ions in the interlayer region of the NAZ samples. Finally, the chromium(VI) ions adsorbed by the NAZ samples were easily desorbed using a desorption solution. The results showed that NAZ offers great potential for the removal of chromium(VI) ions from aqueous solutions.

In this paper, we demonstrated a fluorescent o-phenylenediamine–m-phenylenediamine copolymer sensing system, which was synthesized by a facile and one-step hydrothermal method. The copolymer was first used as fluorescent probe for the detection of Chromium(VI) ion (Cr 6+ ) and showed high selectivity and sensitivity. The detection limit was 1×10 −11 M. It showed excellent linear relationships in wide range of 7×10 −11 –6×10 −10 M. Moreover, the addition of ethylenediaminetetraacetate (EDTA) to the detection system could successfully combine with Cr 6+ to form metal chelates, making the fluorescence recovery of o-phenylenediamine–m-phenylenediamine copolymer. What is important, the prepared process had no addition of initiating agent. [ABSTRACT FROM AUTHOR]

A colloidal silicate granulated nickel-aluminum-zirconium (CSG-NAZ) was prepared, and the chromium(VI) (Cr(VI)) ions recovery capacity was evaluated using a sodium sulfate solution in a column experiment. The amount adsorbed and breakthrough time were enhanced by decreasing the flow rate (flow rate is in the order of 3.0 > 2.0 > 0.5 mL). The breakthrough curves and model parameters were estimated using the Thomas and Yoon-Nelson models. The obtained data confirmed to fit both the Yoon-Nelson model (0.858-0.906) and the Thomas model (0.813-0.906). Additionally, Cr(VI) ions that adsorbed onto CSG-NAZ could be desorbed using a sodium sulfate solution in a column experiment. The total recovery percentage of Cr(VI) ions was 80.9% after six repetitions of adsorption/desorption. Finally, the obtained results revealed that CSG-NAZ was a candidate adsorbent for the recovery of Cr(VI) ions owing to its applicability toward a continuous system.

In this study, we evaluate the desorption or recovery capacity of chromium(VI) ions using desorption solutions containing sodium hydroxide (NaOH) or sodium sulfate (Na 2 SO 4 ). A complex hydroxide of nickel-aluminum-zirconium (NAZ) was prepared as the adsorbent for the removal of chromium(VI) ions. The results from repeated adsorption/desorption experiments on chromium(VI) ions using NAZ complex hydroxide were evaluated. The desorption percentage of chromium(VI) ions increased with the increase in the concentration of NaOH or Na 2 SO 4 in the desorption solution. The determined optimal concentration of NaOH or Na 2 SO 4 in the desorption solution was 10 mmol/L under the used experimental conditions. After three adsorption-desorption cycles, the recovery percentages of chromium(VI) ions using NaOH and Na 2 SO 4 were 60% (total amounts adsorbed and desorbed were 102 and 61 mg/g, respectively) and 75% (total amounts adsorbed and desorbed were 96 and 72 mg/g, respectively), respectively. Additionally, we confirmed the existence of chromium on the surface of the NAZ complex hydroxide. After three adsorption/desorption cycles, the crystal structure of the NAZ complex hydroxide was maintained. These results indicated the potential of the NAZ complex hydroxide using a desorption solution containing NaOH or Na 2 SO 4 for the recovery of chromium(VI) ions.

This paper reports on enzyme-like catalytic properties of polyethylene glycol-functionalized poly(N-phenylglycine) (PNPG-PEG) nanoparticles, which have not been explored to date. The developed nanoparticles have the ability to display great inherent peroxidase-like activity at very low concentrations, and are able to catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) substrate in presence of hydrogen peroxide (H 2 O 2). The oxidized product of TMB has a deep blue color with a maximum absorbance at ~655 nm. The PNPG-PEG nanoparticles exhibit K m values of 0.2828 for TMB and 0.0799 for H 2 O 2 , indicating that TMB oxidation takes place at lower concentration of H 2 O 2 in comparison to other nanozymes. Based on the known mechanism of H 2 O 2 oxidation by hexavalent chromium [Cr(VI)] ions to generate hydroxyl radicals (•OH), these nanoparticles were successfully applied for the colorimetric sensing of Cr(VI) ions. The sensor achieved good performance for Cr(VI) sensing with detection limits of 0.012 μM (0.01–0.1 μM linear range) and 0.52 μM (0.05–12.5 μM linear range). The detection scheme was highly selective, and successfully applied for the detection of Cr(VI) in real water samples. Image 1 • A water dispersible PNPG-PEG nanoparticles are synthesized under mild conditions. • PNPG-PEG displays an enzyme-like activity towards TMB oxidation in presence of H 2 O 2. • The enzyme-like activity is enhanced in the presence of Cr(VI) ions. • A highly selective and sensitive Cr(VI) detection is achieved. [ABSTRACT FROM AUTHOR]

• Amount of chromium(VI) ion adsorbed was related to the physicochemical properties. • Chromium(VI) ion adsorbed onto WB 1000 was desorbed using sodium hydroxide. • WB 1000 could be useful for removal and recovery of chromium(VI) ions. Virgin wheat bran (WB) and calcined wheat bran were prepared at 500 or 1000 °C (WB 500 or WB 1000 , respectively), and their physicochemical properties (such as morphological, thermal, specific surface area, point of zero charge pH, and surface functional groups) were investigated to assess their chromium(VI) ion adsorption capability. The amount of chromium(VI) ion adsorbed was in the order WB < WB 500 < WB 1000. These results showed that the chromium(VI) ion adsorption was related to the WB surface characteristics. Additionally, the amount of chromium (Cr) and oxygen (O) on the WB 1000 surface increased after adsorption, which indicated chromium(VI) ions were adsorbed onto the WB 1000 surface. Adsorption isotherm and adsorption kinetic data fit the Freundlich (0.879–0.991) and pseudo-second-order models (0.979–0.997), respectively. The optimal pH condition for the removal of chromium(VI) ion from aqueous solution was approximately 2. Finally, WB 1000 could be useful for repetition of chromium(VI) ion adsorption/desorption using sodium hydroxide at 1000 mmol/L (at least five times). It was shown that WB 1000 has the potential for adsorption and recovery of chromium(VI) ion from aqueous solution using sodium hydroxide. [ABSTRACT FROM AUTHOR]

In this study, nickel-aluminium complex hydroxides at different molar ratios (nickel-aluminium = 1 : 2, 1 : 1, 2 : 1, 3 : 1, and 4 : 1, referred to as NA12, NA11, NA21, NA31, and NA41) were prepared, and their adsorption capability for chromium(VI) ion was investigated. Firstly, physicochemical characteristics (SEM images, X-ray diffraction (XRD) patterns, specific surface area, amount of hydroxyl groups, and surface pH) of nickel-aluminum complex hydroxide were evaluated. The amount of chromium(VI) ion adsorbed onto NA11 (15.3 mg/g) was greater than that adsorbed onto the other adsorbents. This research elucidated that the amount of chromium(VI) ion adsorbed using nickel-aluminium complex hydroxide was related to the adsorbent surface properties (r = 0.818-0.875). Subsequently, the adsorbent (NA11) surface before and after adsorption of chromium(VI) ion was evaluated, and chromium energy (577 and 586 eV) detected after adsorption onto the NA11 surface. These results revealed that the NA11 surface properties were very important for the removal of chromium(VI) ion from aqueous solution. In addition, the effects of pH, contact time, and temperature on the adsorption of chromium(VI) ion were evaluated. We confirmed a high recovery percentage of chromium(VI) ion when using sodium hydroxide solution at 10-1000 mmol/L (approximately greater than 80%) in this experimental condition. Thus, NA11 is a promising adsorbent for the removal of chromium(VI) ion from aqueous solution.