Photocatalytic oxidation degradation of cyanide is an effective technology to treat cyanide wastewater. TiO₂/SiO₂ is a common catalyst. A preliminary research indicates that the pore structure of SiO₂ has a certain influence on the degradation of cyanide and the removal of zinc and copper. If TiO₂ particles are only loaded on the surface of mesoporous SiO₂, the contact between the surface of SiO₂ and the target substance in wastewater is reduced, thus affecting its adsorption performance. In this study, a macroporous SiO₂ was prepared, and TiO₂ particles were loaded on the surface and in three-dimensional interconnected channels of SiO₂, thus increasing the adsorption sites on the surface of SiO₂ and improving the adsorption performance of target pollutants, and enhancing its efficiency of treating cyanide in wastewater.

Gravity self-assembled polystyrene microspheres (PS) were used as hard templates, impregnated in silica sol for 8.0 h, dried in an oven at 60 ℃, and roasted in a Muffle furnace at 550 ℃ for 10 h to obtain macroporous SiO₂. SiO₂/TiO₂ precursors were prepared by a sol-gel method and roasted at 500 ℃ for 90 min. SiO₂/TiO₂ composites with a three-dimensional interconnected macroporous structure were prepared. The wastewater and materials were placed and mixed in a beaker in a certain proportion. After stirring magnetically for a period of time, a small air pump was used to pump air into the bottom of the beaker at room temperature, and a high-pressure mercury lamp was used as the light source for photocatalytic oxidation of the wastewater. The effects of TiO₂ particle loading mass, catalyst dosage, catalytic time, adsorption time on the treatment of cyanide wastewater were investigated. N₂ adsorption-desorption, scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR) and other characterization techniques were used to determine the structural characteristics of the material and its adsorption-catalytic degradation mechanism.

PS presents a monodisperse, independent and spherical structure with a particle size of 1.65 μm. PS templates are arranged in an orderly manner, independent of each other but closely connected, and they do not show a large continuous structure. SiO₂ is a three-dimensional ordered macroporous structure with a pore size of 1.2 μm, which is smaller than the diameter of microspheres, indicating that the skeleton structure shrinks during the roasting process. SiO₂ particle is broken mainly because of mechanical grinding. The long distance of the channels is orderly, the large holes are uniform and interconnected through small holes. The sizes of the small holes are 170-220 nm, the holes are formed due to the accumulation of microspheres. TiO₂ particles are fragmented and loaded on the concave surface of SiO₂. The particle size of TiO₂ is uneven, and smaller than the pore size of SiO₂. TiO₂ particles are evenly dispersed on the surface of SiO₂ or in the physical structure of the cavity, which eliminates the agglomeration of TiO₂. N₂ adsorption-desorption test shows that the SiO₂/TiO₂ material has a large pore structure, and the specific surface area is increased by 5 times, compared with TiO₂ nanoparticles. The thermogravimetric curve shows that the materials have less weight loss at 250-800 ℃. The X-ray diffraction pattern of the material shows that the diffraction peak intensity of SiO₂ loading with TiO₂ particles decreases. The experimental results show that the degradation efficiency of total cyanide and the removal efficiencies of copper, iron and zinc are 98.79%, 99.10%, 100.00% and 92.26%, respectively, under the optimum conditions (i.e., 10% of TiO₂ particle loading mass, 0.35 g of the material mass when treating 100 mL cyanide wastewater, 1.0 h of dark adsorption and 4.0 h of illumination time). The material was regenerated in a sulfuric acid solution by ultrasonic treatment. Under the same experimental conditions, the total cyanide degradation efficiency is reduced by 4.95% after three catalytic cycles, indicating that the material has strong stability and reusability. The XRD results before and after adsorption-catalysis show that ZnO, CuSCN, Cu₂Fe(CN)₆ and Fe₂O₃ are adsorbed on the surface of the material. The FTIR results show that a characteristic peak of C≡N appears on the surface of the material after 1-h adsorption and a characteristic peak of NO₃^- appears after 4-h catalysis. The EDS of the material after 1-h adsorption shows that cyanide is adsorbed on the surface of the material, and metal ions are uniformly distributed on the surface of the material. The EDS of the material after 4-h illumination shows that the metal content is higher than that after 1-h adsorption, and the distribution of Si and Ti is uniformly crossed. The results of free radical quenching experiment show that electron capture by oxygen is the main factor to improve the catalytic effect.

Macroporous SiO₂/TiO₂ materials were prepared by PS self-assembly template, impregnation, roasting and loading, which were used to treat cyanide wastewater. Under optimal experimental conditions, the degradation efficiency of total cyanide and the removal efficiencies of copper, iron and zinc were 98.79%, 99.10%, 100.00% and 92.26%, respectively. The three-dimensional ordered pore structure of macroporous SiO₂ made TiO₂ particles evenly dispersing on its surface and inside the cross-linked pore. The mechanism indicated that the cyanide was degraded into non-toxic nitrogen oxides and carbon oxides, and the metals were removed by precipitation.