DOI: 10.14457/tu.the.2016.516
title: Nanosorption-membrane filtration for heavy metal removal

description: A novel technique has been developed for heavy metal removal. This allows a simultaneous filtration and adsorption by coating modified adsorbent onto supporting materials which lead to shorten treatment process and improve the adsorption capacity for heavy metal removal. New biosorbent in nanoscale was prepared and investigated for its efficiency for heavy metal removal. Chitosan nanoparticles (CN) was prepared by using ionic gelation method. The adsorbent was introduced onto the supporting material using a dip coating method. Moreover, CN were modified using glutaraldehyde (GLA), and polyethylene glycol (PEG) in order to increase the stability and binding properties. The size and morphology of adsorbents and supporting materials were characterized by using an atomic force microscope (AFM) and scanning electron microscope (SEM). The element and chemical analysis was examined by energy dispersive spectrometer (EDS). Interaction between components of adsorbent and supporting material were also characterized by Fourier transform infrared spectrometer (FTIR). The experiments were performed under batch and continuous system at room temperature. The mechanism of adsorption was studied using adsorption isotherms and adsorption kinetics. The efficiency of removal was tested by three different heavy metals which are Cr(VI), Cu(II), and Ni(II). The dry sizes of CN were in the range of 23-26 nm based on AFM examination. SEM images showed significant changes on the surface morphology from a coarse to a smooth uniform surface of film-like morphology. The film became thicker and smoother as the CN loading increased. FTIR spectra showed no interaction between the CN and the supporting material. The EDS composition before and after heavy metal adsorption provided direct evidence for heavy metal adsorption by the adsorbent.In batch experiments, bituminous activated carbon (AC) and ceramic balls (CB) were used as supporting material for Cr(VI) and Cu(II) removal. The experiments were carried out to investigate the influences of several operational parameters such as contact time, adsorbent dose, agitation rate, pH, and initial concentration. The CN-coated AC demonstrated an increase in Cr(VI) removal efficiency and also in adsorption capacity. The adsorption capacity of the CN-coated AC (77.52 mg/g) was more than twice that of the uncoated AC (36.36 mg/g) or pure chitosan (32.57 mg/g) at pH 5. CN-coated CB showed difference adsorption capacity between Cr(VI) and Cu(II). The adsorption capacity for Cu(II) was more than three times as compared to the Cr(VI) adsorption. The adsorption isotherms were well described using the Freundlich and Langmuir models. Adsorption kinetics followed that of the pseudo-second-order kinetics, suggesting chemisorption as a rate limiting step. For the continuous system, the experiments were performed to study the effect of adsorbent dosage, pH, initial concentration, and flow rate. CB and tubular microfiltration ceramic membrane (CM) were used as supporting materials for Cu(II) and Ni(II) removals. The performance of CN-coated CB for Cu(II) removal through a fixed-bed column was evaluated. The adsorption capacity increased with increasing the initial concentrations, but decreased with increasing flow rate and CN loading (187.4 mg/g). The Thomas and Yoon-Nelson models were shown to have excellent fit for the breakthrough curves, indicating that CN-coated CB is suitable for a columnar design. The efficiency for Cr(VI) removal was low. Thus, only Cu(II) and Ni(II) were investigated in the continuous system. The CN was introduced onto the CM surface (CN-coated CM) for Cu(II) and Ni(II) removal through a continuous system of simultaneous filtration and adsorption method. CN crosslinked with GLA provided the highest adsorption capacity of 240.3 mg/g at flow rate 2.5 mL/min.  It is over 30% improvement of adsorption capacity as compared to native CN. However, the reduction of percentage and adsorption capacity can be attributed to a short contact time insufficient for Cu(II) diffusion into the inner pores of adsorbent. For Ni(II) removal, the adsorption capacity was 123.0 mg/g. The amount of Ni(II) removed increased with an increase in flow rate due to larger flow, but decreased with an increase in the amount of CN loading, indicating that the adsorption takes place mainly on the surface of CN film which, in turn, is controlled by surface area of the membrane. The adsorption kinetics were well described using the Thomas model for both Cu(II) and Ni(II). The permeate flux of all conditions slightly decreased and then the plateau with time. The permeate flux decreases because of the thicker CN layer on the membrane surface and the concentration polarization. This tendency showed that interaction between the CM and CN led to the formation of the CN layer on the surface of CM and induced a reduction of the membrane pore size. The simultaneously filtration and adsorption experiments exhibited a high adsorption capacity for Cu(II) removal by using CN crosslinked with GLA (CMCN-GLA) coated membrane at pH 5 and the flowrate 2.5 mL/min. The results are useful for further applications of tertiary wastewater treatment system design at low concentration of heavy metals from industrial effluents prior to their discharge. The advantage of this method is that heavy metals can be simultaneously treated by filtration of particulate matters whose size is greater than the membrane pore size together with suultaneous adsorption of heavy metals onto the coating surface of the membrane.