An innovative zinc oxide-coated zeolite adsorbent for removal of humic acid
Graphical abstract
Introduction
Humic acids (HA) and fulvic acids are ubiquitous in surface water. They are also found in ground water and sources of drinking water supply. Humic and fulvic acids could bring undesirable color and odor to drinking water [1] and react with chlorine to produce potential carcinogens such as trihalomethanes and haloacetic acids during drinking water treatment [2], [3], [4], [5]. Therefore, it is of great importance to remove HA from sources of drinking water supply.
Adsorption has gained much attention to remove HA from water due to its ease of operation and high efficiency. Furthermore, the adsorption process can remove a large number of toxic substances from the treated water, and has no side effects or secondary pollution [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. However, one process, granular activated carbon (GAC), is generally effective, but not cost effective, thus new adsorbents are being sought. Since the amount of adsorbent required in the adsorption process and the required frequency of regeneration are important parameters for processes to determine operational costs, there is a need to develop a highly efficient adsorbent for the maximum removal of HA from water.
HA is a coiled, long-chain molecule that can contain negatively charged functional groups under typical environmental pH values. The functional groups are mainly derived from the ionization of carboxyl groups, leading to mutual repulsion and expansion of the coil of HA [16]. Therefore, it is important to develop positively charged adsorbents to improve the efficiency of HA removal for adsorption processes [17].
In view of their unique external and internal surface reactivity, zeolites have attracted much attention in the scientific community. Extensive research effort has been undertaken to improve further their physicochemical properties, which are directly associated with the efficiency of zeolites for the removal of HA and other natural organic matter from water [11], [12], [13], [14], [15]. Furthermore, since ZnO could dissociate hydrogen in a heterogeneous manner [18], [19], [20], an appropriate matrix tailored by ZnO will increase the amount of positively charged groups on the matrix. As a result, we chose zeolite 4A as a matrix and developed ZnO-coated zeolite adsorbents. Herein, zeolite 4A was modified by nitric acid (HNO3) treatment and then was functionalized by Zn(NO3)2·6H2O. In addition, we have demonstrated that ZnO-coated zeolite adsorbents could efficiently remove HA from aqueous solution. The adsorption of HA onto ZnO-coated zeolite adsorbents was investigated in batch adsorption experiments in order to determine 1) the optimal amount of ZnO coated on zeolite, 2) the role of ZnO on the effectiveness of HA adsorption onto ZnO-coated zeolite adsorbents, and 3) the effect of initial HA solution pH and ionic strength on the adsorption capacity. We focused on the evaluation of the effect of nitric acid modification on the HA adsorption capacity. ZnO-30-zeolite was produced without nitric acid modification for comparison. Since the evaluation of selective adsorption of ZnO-coated zeolite adsorbents is crucial for their future practical applications in the field of environmental remediation, we investigated the surface properties of ZnO-coated zeolite adsorbents by surface characterization techniques. Finally, the effect of contact time was examined to provide important information on the adsorption mechanism.
Section snippets
Adsorbates
HA was the target contaminant in this study to assess the adsorption capacity of the prepared adsorbent materials. HA was purchased from Sigma-Aldrich (Milwaukee, WI, USA). A stock solution of HA (140 mgC L−1) was prepared by dissolving HA in autoclaved de-ionized water for 48 h and filtered with a membrane filter (pore size: 0.45 μm, EMD Millipore, Germany). The stock solution of HA was stored in the dark at 4 °C before use.
Preparation of ZnO-coated zeolite adsorbents
A portion of commercial zeolite 4A was modified by HNO3 with an optimal
Characterization
Mechanisms contributing to the adsorption of HA on ZnO-coated zeolite adsorbents were explored by surface characterization techniques and adsorption tests. Fig. 1 shows the X-ray diffraction patterns of (a) zeolite, (b) ZnO-10N-zeolite, (c) ZnO-20N-zeolite, and (d) ZnO-30N-zeolite, compared to the standard patterns of zeolite A (JCPDF#38-0241) and ZnO (JCPDF#89-0511). Compared with that of the zeolite 4A, ZnO-10N-zeolite, ZnO-20N-zeolite, and ZnO-30N-zeolite exhibited similar characteristic
Conclusions
ZnO-coated zeolite adsorbents were developed with high adsorption capacity for the removal of HA from aqueous solutions. The highest adsorption capacity was achieved with 30 wt% loading of ZnO on the zeolite. Modification of the ZnO-coated zeolite adsorbents with HNO3 significantly improved their HA adsorption capacity. Structural characterization proved that ZnO particles coated on zeolite and ZnO-coated zeolite adsorbents have higher BET surface area and porosity. This helped to increase
Acknowledgements
The work was financially supported by the the Natural Science Foundation of Fujian Province, P.R. China (No. 2014J01055), and partially funded by the Cyprus Research Promotion Foundation through Desmi 2009-2010 which was co-financed by the European Regional Development Fund and the Republic of Cyprus (Strategic Infrastructure Project ΝΕΑ ΥΠΟΔΟΜΗ/ΣΤΡΑΤΗ/0308/09). C. Han was supported by the Postgraduate Research Program at the National Risk Management Research Laboratory administered by the Oak
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