## 3 Results and Discussion
### The Removal Effect of the Mixed Filter Material
#### 3.2.4 The Cr(VI) Removal Effect  
Figure 10 shows the Cr(VI) removal effect of single volcanic rock and coconut shell activated carbon filter media and their mixed filter media over 36 days. In the early stage of operation, the mixed filter material with a mixing ratio of 1:3 (Reactors 5\({}^{\#}\)) performed the best in the removal of Cr(VI) compared with other mixed filter materials (Reactors 4\({}^{\#}\) and 6\({}^{\#}\)), with a removal rate of 72.19 \(\pm\) 4.93%. Previous studies have found that among single filter media, Reactors 3\({}^{\#}\) has the best removal effect on Cr(VI). Through comparison, it was found that the average removal rate of Reactors 5\({}^{\#}\) was 10.89 percentage points higher than that of Reactors 3\({}^{\#}\). By mixing volcanic rock with coconut shell activated carbon filter material, a multi-level pore structure is formed, enhancing the electrostatic adsorption capacity of the filter material. In addition, the metal cations in volcanic rock interact with the functional groups and reducing substances on the surface of coconut shell activated carbon, forming an efficient mixed reduction reaction system, which further enhances the Cr(VI) removal effect. After 26 days of operation of the BSF reactor, the Cr(VI) removal rate reached a stable state. Reactors 5\({}^{\#}\) had the best removal effect on Cr(VI), which was 0.24 percentage points higher than that of Reactors 3\({}^{\#}\). This difference was masked by the standard deviations of the two, indicating that it did not significantly exceed the performance boundary of a single filter material, which is related to the influence of the uniformity of microbial community distribution.  
Figure 10: The removal effect of mixed filter media on Cr(VI).  
Figure 9: The removal effect of mixed filter media on TOC.  
In this experiment, under the conditions where the rough surface of volcanic rock and the microporous structure of coconut shell activated carbon provided different attachment spaces for microorganisms, the spatial distribution and metabolic mechanism of the microbial community in the mixed filter material were unclear due to the lack of high-throughput sequencing or electron microscopy characterization of the biofilm. The existing results indicate that the improvement in the pollutant removal rate of the mixed filter material is limited. Perhaps due to the insufficient microbial co-metabolism on the surface of the filter material, it is necessary in the future to further explore the association between the structure and function of the microbial community by combining biofilm characterization techniques such as qPCR and FISH.  
Table 7 presents the average pollutant removal rates of BSF Reactors \(1^{\#}\), \(2^{\#}\), \(3^{\#}\), \(4^{\#}\), \(5^{\#}\), and \(6^{\#}\) during the early operation and stable removal periods. The filter media of BSF Reactors \(1^{\#}\), \(2^{\#}\), and \(3^{\#}\) consist of quartz sand, volcanic rock, and coconut shell activated carbon, respectively. In contrast, the filter media of Reactors \(4^{\#}\), \(5^{\#}\), and \(6^{\#}\) are composed of mixed volcanic rock and coconut shell activated carbon, with mixing ratios of (1:1), (1:3), and (3:1), respectively.