Dongwei Li

Chromium(VI) sorption efficiency of acid-activated banana peel over organo-montmorillonite in aqueous solutions

ABSTRACT In the present study, we examined sorption of chromate (Cr(VI)) to acid-activated banana peel (AABP) and organo-montmorillonite (O-mont) as a function of pH, initial Cr(VI) concentration at a sorbent dose of 4 g L−1 and at 20 ± 1°C in aqueous solutions. In sorption edge experiments, maximum Cr(VI) removal was obtained at pH 3 after 2 hours by AABP and O-mont (88% and 69%). Sorption isotherm data showed that the sorption capacity of AABP was higher than O-mont (15.1 vs. 6.67 mg g−1, respectively, at pH 4). Freundlich and Langmuir models provided the best fits to describe Cr(VI) sorption onto AABP (R2 = 0.97) and O-mont (R2 = 0.96). Fourier transform infrared spectroscopy elucidated that for AABP mainly the –OH, –COOH, –NH2, and for O-mont intercalated amines and –OH surface functional groups were involved in Cr(VI) sorption. The scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX) analyses, although partly, indicate that the (wt. %) proportion of cations (e.g., Ca, Mg) in AABP decreased after Cr(VI) sorption. This may be due to ion exchange of chromite (Cr(III)) (produced from Cr(VI) reduction) with cationic elements in AABP. Also, Cr(VI) desorption (using phosphate solution) from AABP was lower (29%) than that from O-mont (51%) up to the third regeneration cycle. This bench scale comparative study highlights that the utilization of widely available and low-cost acid-activated biomaterials has a greater potential than organo-clays for Cr(VI) removal in aqueous media. However, future studies are warranted to precisely delineate different mechanisms of Cr(VI) sorption/reduction by acid-activated biomaterials and organo-clays.

Heavy metal removal from MSWI fly ash by electrokinetic remediation coupled with a permeable activated charcoal reactive barrier.

This paper presents the investigations into the feasibility of the application of a remediation system that couples electrokinetic remediation (EKR) with the permeable reactive barrier (PRB) concept for municipal solid waste incineration (MSWI) fly ash with activated charcoal as the PRB material. The experimental results of this study showed that the proposed combined method can effectively improve the remediation efficiency and that the addition of the oxalic acid to the PRB media before the coupled system can further enhance the remediation process. In the optimization tests, the maximum removals of Zn, Pb, Cu and Cd were achieved under different experimental conditions. The voltage gradient and processing time were shown to have significant effects on the removal of Cu and Cd, whereas the addition of the oxalic acid had a more significant influence on the removal of Pb. Generally, the processing time is the most significant factor in changing the removal rates of HMs in the enhanced coupled system. In terms of the leaching toxicity, the specimen remediated by ENEKR + PRB showed the lowest leaching value for each HM in the S2 and S3 regions.

The Detoxification of Heavy Metals in the Phosphate Tailing-contaminated Soil through Sequential Microbial Pretreatment and Electrokinetic Remediation

ABSTRACT In this paper, the heavy metals (HMs) in the phosphate tailing-contaminated soil were detoxified using the microbial pretreatment in combination with electrokinetic remediation (EKR).. This study provides compelling evidence that the sequential usage of the bioleaching and electrokinetics is superior to the individual method for the detoxification of Pb, Cu, Zn, Cd, and As from the contaminated soil. In the sequential system, the detoxification efficiency of Zn was the highest and that of As was the lowest. Except the element As, the detoxification efficiencies of HMs in the sequential system were generally higher than that using the single biological treatment and EKR technique. Bioleaching, generation of the passivation, and migration direction of the ions are concluded as the factors attributable to the final results; and, the initial increase in the inoculation doping of Thiobacillus ferrooxidans considered has no obvious impact on improving the final detoxification efficiency rates.

Study on electro-kinetic remediation of heavy metals in municipal solid waste incineration fly ash with a three-dimensional electrode

Fly ash from incinerated municipal solid waste is a source of secondary pollutants (Cu, Zn, Pb and Cd). The inappropriate management of fly ash leads to the contamination of soil and ground water. This paper is based on the electro-kinetic removal of heavy metals from municipal solid waste incinerated fly ash by using a three-dimensional electrode in orthogonal single and multi-factor experiments to obtain the optimal experimental conditions by varying the leaching toxicity removal rate for the heavy metals in the groups. The optimal dimensions (6 × 6 mm) of the particle electrodes to achieve high removal rates for heavy metals are found by using various measurements in single-factor orthogonal experiments. In addition, the multi-factor orthogonal experiment is based on three factors: (a) the particle electrode ratio, (b) the voltage gradient and (c) the repair time, while keeping the optimum specification (6 × 6 mm) for particle electrodes constant. The result showed that a high removal rate for heavy metals was obtained by applying a voltage gradient of 9 V for 5 days (repair time) and a 5% dosing ratio.

Near-anode focusing phenomenon caused by the coupling effect of early precipitation and backward electromigration in electrokinetic remediation of MSWI fly ashes

As electrokinetic remediation (EKR) is used to remediate contaminated soil, the focusing phenomenon is always the inevitable problems needing to be solved during the process. In this paper, EKR was chosen to extract the heavy metals in the municipal solid waste incineration fly ashes (FAs). The focusing phenomenon of FA in the sample region of the rectangular electrolyser was comprehensively studied. The formation of the focusing band was monitored and the redistribution of concentration for Zn, Pb, Cu, and Cd were displayed. The major and minor phases of FA particles have greatly changed before and after the experiment. A variety of carbonate hydroxides and phosphate were found at the end of the test. There are two focusing bands at S2 and S5, separately for all the four elements and the focusing bands in the S2 zone forming at the early stage existed nearly throughout the entire process. The pH gradient between S1 and S2 was detected at the onset of the experiment, and until the 12 day, the gradient barrier was obviously dropped down. The coupling effect of precipitation hindering and backward electromigration of anions is considered as the dominant force for the formation of the focusing band, which is different from the pure ion-induced potential gradient well trapping effect near the anode. The FA matrix enhancing and nitric acid enhancing can alleviate the focusing phenomenon and increase the concentration ratios to some extent.

Solidification/stabilization of chromite ore processing residue using alkali-activated composite cementitious materials

Chromite Ore Processing Residue (COPR) produced in chromium salt production process causes a great health and environmental risk with Cr(VI) leaching. The solidification/stabilization (S/S) of COPR using alkali-activated blast furnace slag (BFS) and fly ash (FA) based cementitious material was investigated in this study. The optimum percentage of BFS and FA for preparing the alkali-activated BFS-FA binder had been studied. COPR was used to replace the amount of BFS-FA or ordinary Portland cement (OPC) for the preparation of the cementitious materials, respectively. The immobilization effect of the alkali-activated BFS-FA binder on COPR was much better than that of OPC based cementitious material. The potential for reusing the final treatment product as a readily available construction material was evaluated. X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR) and scanning electron microscope with energy dispersive spectrometer (SEM-EDS) analysis indicated that COPR had been effectively immobilized. The solidification mechanism is the combined effect of reduction, ion exchange, precipitation, adsorption and physical fixation in the alkali-activated composite cementitious material.

The application of homemade Neosinocalamus affinis AC in electrokinetic removal technology on heavy metal removal from the MSWI fly ash

This present paper was focused on the manufacture of activated carbon (AC) and its application in the electrokinetic remediation (EKR) technology on removal of the heavy metals (HMs) from the municipal solid waste incineration fly ash. AC was produced from Neosinocalamus affinis (NF) by chemical activation with H3PO4 in N2 atmosphere, the effects of activation temperatures, soaking time and impregnation ratios on the adsorption capacity of AC on HMs were examined through equilibrium adsorption experiments. The AC produced under the condition of 450 °C of activation temperature, 10 h of soaking time and 1.5 of impregnation ration was applied in the EKR experiment. The addition of AC in the S3-region of the electrolyzer could effectively improve the removal efficiencies of HMs. The technical parameters of voltage gradient, processing time and proportion were further optimized in the coupled experiments, the maximum removal of Cu, Zn, Cd, and Pb was 84.93%, 69.61%, 79.57%, and 78.55% respectively obtained under the optimal operating conditions of 2 V/cm of voltage gradient, 8 d of processing time and 20% of proportion.

Application of iron-loaded activated carbon electrodes for electrokinetic remediation of chromium-contaminated soil in a three-dimensional electrode system

Hexavalent chromium from industrial residues is highly mobile in soil and can lead to the contamination of groundwater through runoff and leaching after rainfall. This paper focuses on the three-dimensional (3D) electrokinetic remediation (EKR) of chromium-contaminated soil from an industrial site. Activated carbon particles coupled with Fe ions (AC-Fe) were used as the third electrode. The optimum dose ratio of the electrode particles and remediation time were selected on the basis of single-factor experiments. X-ray photoelectron spectroscopy (XPS) analysis was carried out to explore the reduction of Cr(VI) on the surface of the electrode particles (AC-Fe). The results showed that AC-Fe had a positive effect on Cr(VI) reduction with a removal rate of 80.2%, which was achieved after 10 d by using a 5% dose of electrode particles. Finally, it was concluded that the removal mechanism combined the processes of electromigration, electrosorption/adsorption and reduction of Cr(VI) in the 3D EKR system.

Electrokinetic remediation of heavy metals from municipal solid waste incineration fly ash pretreated by nitric acid

Municipal solid waste incineration (MSWI) fly ash has a high concentration of heavy metals (HMs) which are hazardous to the environment. Moreover, it has high pH and buffering capacity which hinders the removal of HMs. Another constraining factor is the considerable fraction of HMs which exist in oxidizable and reducible states. The acid pretreatment of MSWI fly ash is a key solution to this problem. Therefore, the current experiment is carried out to evaluate the effect of acid pretreatment of MSWI fly ash and reaction/proposed time on the removal efficiency of HMs through an electrokinetic experiment. The leaching experiment results show that acid pretreatment has increased the desorption/release of heavy metal ions (Pb2+, Cd2+, Cu2+ and Zn2+). It enhances the migration of HM ions in electrolytic cells which get precipitated at the cathode, thereby increasing the removal efficiency of HMs in the electrokinetic experiment. Moreover, it is found that prolonged proposed time (12 d) has significant effect on the removal efficiency of HMs. Finally, it is concluded that acid pretreatment and prolonged proposed time have enhanced the removal electrokinetic remediation of HMs which is carried out via three processes, i.e. desorption (enhanced by acidification), migration and precipitation.

Waste solidification/stabilization of lead–zinc slag by utilizing fly ash based geopolymers

Solidification/stabilization (S/S) is recognized as an effective technology for solid waste treatment. In S/S, the application of geopolymers synthesized by industrial waste (rich in active silicon and aluminum) to immobilize hazardous waste is a research focus. In this article, a fly ash based geopolymer was used to immobilize lead–zinc slag containing Pb, Ni, Zn and Mn. A fly ash based geopolymer with good mechanical strength was obtained through single factor experiments and the compressive strength of the geopolymer reached 29.72 MPa. The effects of immobilizing lead–zinc slag in the fly ash based geopolymer were discussed by means of compressive strength, leaching test and speciation analysis. The solidification/stabilization mechanism was further investigated using XRD, FTIR and SEM. The mechanical properties of the fly ash based geopolymer were negatively affected by addition of lead–zinc slag, and compressive strength decreased to 8.67 MPa when 60% lead–zinc slag was added. The geopolymer has the ability to reduce toxicity of lead–zinc slag by immobilizing heavy metals (Pb, Ni, Zn and Mn), but the ability was not unlimited. The migration of heavy metals to residual form indicates that heavy metals may either be bonded into the geopolymer matrix via the T–O bond (T = Si, Al) or captured in framework cavities to maintain the charge balance. The NASH (Na2O–Al2O3–SiO2–H2O) gel structure observed by XRD, FTIR and SEM can physically encapsulate the contaminants during geopolymerization. It is finally concluded that heavy metals were immobilized in the fly ash based geopolymer through a combination of chemical bonding and physical encapsulation.