In the past several decades, nano-materials and technologies for advanced water treatment have attracted a lot of attention in academic and industrial circles. In the actual water treatment process, nano-materials often have bottlenecks such as easy agglomeration and deactivation, difficult operation, potential safety risks, etc. One of the most effective strategies to deal with these bottlenecks is by using large-size inert carriers to fix nano-particles and prepare composite nano-materials with confined structure. Studies in chemistry, materials science and other related disciplines show that nano-materials after confinement are often significantly different from bulk materials in structure and performance[1, 2], that is, nano confinement effect. This effect has also been partially confirmed in the research of advanced water treatment, but the scientific mechanism research on confinement effect of composite nano-materials in water treatment is just in its infancy.
Advanced oxidation technology based on persulfate activation (PS-AOP) has attracted much attention because of its high removal efficiency, simple operation and strong versatility[4–6]. Many studies have shown that hydroxyl radicals and sulfate radicals are the key species to activate persulfate (PS) systems. However, PS-AOP technology still faces challenges such as low selectivity of target pollutants removal and difficult catalyst recovery, both of which limit the practical application of PS-AOP. Therefore, it is necessary to explore a suitable catalyst carrier to solve the problems of catalyst recovery and surface area increase. Because of its three-dimensional channel and good thermal conductivity, metal foam has attracted extensive attention for use as catalyst carriers in various fields [8–10].
Ever increasing number of research papers on activation and application of persulfate indicate that sulfate-radical (SO4•−) based Advanced Oxidation Processes (SR-AOP) have been widely studied in recent years. Persulfate activation can generate sulfate-radicals through ultraviolet irradiation, heat or metals, and the latter constitutes a new oxidation technology[11, 12]. Although cobalt/PMS systems were proved to be the most effective way to produce sulfate-radicals[6, 13], cobalt with high toxicity was easy to leach out in homogeneous and heterogeneous reactions, which is a disadvantage as it might cause secondary pollution. Therefore, the application of sulfate radicals produced by non-cobalt-based heterogeneous catalysts in environmental remediation is particularly useful.
Polyvalent manganese oxides are abundant in natural environments such as soil, ocean, rocks and fresh water, usually in the form of oxides of manganese (II), manganese (III) and manganese (IV), which are environmentally friendly and far less toxic than cobalt ions. Manganese oxide has the advantages of low cost, easy availability, good environmental compatibility, high catalytic activity and so on, and is expected to be a substitute for cobalt-based catalysts. Ni is the ninth most abundant element in the Earth’s crust and has a lower toxicity than Co, and hence Ni-based materials have attracted special attention to activate PS or PMS and proved to be efficient catalysts. The synthesis method of nickel hydroxide (Ni(OH)2) is simple and has a broad application prospect in catalytic system. Yue et al. first demonstrated the role of α-Ni(OH)2 as a highly effective nickel-based heterogeneous catalyst for sulfate radical activation.
The monolithic catalyst with three-dimensional hierarchical structure can not only keep the bulk materials easy to separate and reduce the secondary pollution caused by leaching of metal ions, but also avoid the agglomeration of micro/nano particles and expose more active sites, thus significantly improving the catalytic performance. Among many porous foam materials, nickel foam (NF) has high application value because of its unique advantages, including three-dimensional interconnected texture, outstanding mechanical firmness, heat resistance, low economic cost and good environmental stability[19, 20].
As far as we know, previously reported Ni(OH)2 or MnO2/Ni(OH)2 Nanomaterials on NF were mainly designed for supercapacitors[21–23], but the catalytic performance of hybrids for persulfate has never been investigated. Herein, we report a very simple, low temperature and green route to prepare Ni(OH)2 nanosheets and MnO2 nano flowers at the same time on the surface of NF by in situ oxidation-reduction method. The morphology, crystal structure and specific surface area of the catalyst were characterized. The obtained catalyst showed excellent activity and good stability in peroxodisulfate-based Fenton-like reactions. In addition, the mechanism of catalytic oxidation of OII by MnO2/Ni(OH)2/NF/PS heterogeneous system was further studied.