Removing or decreasing hexavalent Chromium from wastewater to the permitted levels is important due to its non-biodegradation, bioaccumulation, cancer-causing and toxic effects. In this study, biosorption of Cr(VI) from aqueous solutions by Excess Active Municipal Sludge was investigated as a function of initial Chromium (VI) concentration (in the range of 5-90 mg/l), initial pH (in the range of 2-8), agitation speed (in the range of 50-200 rpm), adsorbent dosage (in the range of 2-10 g/l) and agitation time (in the range of 5-480 min) in a batch system. The optimum conditions were found by full factorial design approach. The results showed that the equilibrium time for adsorbent is 120 minutes. Also, sorption data have a good fitness by Freundlich isotherm model and adsorption kinetic is adopted with pseudo-second order model. In batch studies, at optimum condition (90 mg/l initial concentration, pH 2, agitation speed 200 rpm and adsorbent dosage 4 g/l), the adsorption performance was about 96%; the maximum adsorption capacity was calculated about 41.69 mg of Cr/g of adsorbent. Overall, it can be concluded that Excess Active Municipal Sludge, has a good performance as a biological, biodegradable, abundant and low-cost adsorbent for the removal of Cr (VI) from aqueous solutions.

Well-dispersed magnetite nanoparticles of different sizes between 15 and 43 nm were synthesized by an electrochemical method in a controlled manner by simply changing the synthesis temperature. These nanoparticles were used as a reusable adsorbent to remove Cr (VI) from aqueous solution. The recovery efficiency was found to be highly dependent on environmental parameters, such as temperature and pH. In addition, it was demonstrated that the initial concentrations of both Cr (VI) and magnetite nanoparticles strongly influence the removal capability of these nanomaterials. Remarkably, the nanoparticle size has a key role on the Cr (VI) adsorption efficiency, which gradually increases as the diameter decreases due to the augmentation of the surface area. The low aggregation in the case of small particles that results from the low magnetization saturation values also contributes to the enhancement of the surface area available for Cr (VI) adsorption. In contrast, nanoparticles with larger sizes are more easily manipulated by a magnet, and the efficiency is largely maintained after five cycles. The adsorption process fits the Langmuir isotherm model well, and the reaction was found to follow a pseudo-second-order rate.

The powdered activated carbon (AC) supported by carbonaceous nano-adsorbents were examined to remove hexavalent chromium [Cr (VI)] from aqueous solution The adsorption behaviour of micro-level concentration of Cr (VI) on those nano-adsorbents was investigated as a function of the experimental conditions such as the contact time, the pH, the dosage of adsorbent, and the initial concentration of Cr (VI). The structural characterization of the adsorbents was accomplished by Fourier transform infrared spectroscopy and scanning electron microscopy.Adsorption isotherms including Freundlich and Langmuir have been applied to study the equilibrium of the adsorption behaviour and identify the adsorption capacity of the activated carbon-functionalized multiwalled carbon nanotubes (AC/f-MWCNTs) and activated carbon-functionalized carbon nanospheres (AC/f-CNSs). Langmuir isotherm model showed that the adsorption process was monolayer type under working with an adsorption capacity of 113.29 and 105.48 mg/g, respectively, for AC/f-MWCNTs and (AC/f-CNSs).

The Cr (VI) adsorption characteristics of Ulmus leaves (UL) and their ash were examined as a function of contact time, initial pH, and initial metal ion concentration. Batch adsorption experiments were performed. The effects of Ca2+, Mg2+, Na+ and K+ on adsorption were studied. Maximum removal was achieved in the pH=7, contact time of 60 minutes and with initial Cr (VI) concentration of 2 mg/L. Because at pH ≥6.5, precipitation of chromium may take place, optimum pH was selected at 6. Maximum adsorption rate values were found at the first 60 min at pH= 6. The effect of the adsorbent on COD removal from aqueous solution showed that 2g/L of adsorbent caused 130 and 75 mg /L increase in COD of deionized water in 60 min for UL and its ash, respectively. The chromium adsorption data obtained under the optimum condition were described by the Langmuir and Freundlich isotherms. Studies showed that the Langmuir adsorption model better fitted than Freundlich, with R2 >0.99. Also UL ash was more efficient than living leaves in removing chromium from aqueous solution.

Metal biosorption on plant-based materials and agricultural wastes is a well practiced but a complex process affected by several factors. The biosorption of chromium(VI) from aqueous solution onto waste plant biomass of Portulaca Oleracea was studied in the present work. Batch studies were carried out to examine the effects of process parameters. Influence of altering various process parameters was studied. The biosorption process was fast, and equilibrium was achieved in 45 min of contact time. It was found that the biosorption capacity of plant material depends on many factors mainly on solution pH, with a maximum biosorption capacity for chromium at pH 2. The biosorption kinetics was tested with pseudo-first-order and pseudo-second-order reaction, and results showed that biosorption followed pseudo-second-order rate expression. Experimental equilibrium data were applied to two different isotherm models. Isotherm tests showed that equilibrium sorption data were better represented by Langmuir model, and the sorption capacity of plant biomass was found to be 54.945 mg/g. Thermodynamic parameters like DG 0, DH 0 and DS 0 were also evaluated, and it was found that the biosorption was spontaneous and endothermic in nature. Plant biomass was found to be an effective adsorbent for chromium (VI) from aqueous solution. This study indicated that plant biomass could be used as an efficient, cost-effective and environmentally safe biosorbent for the treatment of chromium containing aqueous solutions.