## 3 Results and Discussion
### The Removal Effect of a Single Filter Material
#### 3.1.4 The Cr(VI) Removal Effect  
Figure 6 presents the Cr(VI) removal effect of a single filter material over 36 days. In the initial operation stage, due to limited biological growth, Cr(VI) removal primarily relies on the physical and chemical effects of the filter material, including electrostatic adsorption and the chemical reduction of Cr(VI) to Cr(III) through the interactions of metal cations on the surface of volcanic rock, functional groups on the surface of coconut shell activated carbon, and reducing substances. Subsequently, a chromium hydroxide precipitate is formed [30; 31]. Among them, the Cr(VI) removal rates of coconut shell activated carbon (61.30 \(\pm\) 5.37%) and volcanic rock (42.29 \(\pm\) 5.00%) were significantly better than that of quartz sand (14.56 \(\pm\) 6.09%) (\(p\) < 0.05). Meanwhile, it was observed that coconut shell activated carbon had the best removal effect on Cr(VI), with an average removal rate of  
Figure 5: The removal effect of a single filter material on TOC.  
19.01 percentage points higher than that of volcanic rock. This shows that in the initial stage of operation, coconut shell activated carbon had the best removal effect on Cr(VI) because of its high specific surface area and surface reducing substances, followed by volcanic rock.  
As the system's operation time increased, the degradation effect of microorganisms intensified. Some facultative anaerobic microorganisms (such as Bacillus and Pseudomonas) participate in the reduction process of Cr(VI) through enzymatic reactions. Under anaerobic conditions, these microorganisms can use Cr(VI) as an electron acceptor, further enhancing the removal rate [32]. Studies have shown that this type of microorganism can secrete reductase, which is highly active in neutral to warm environments with a pH of 6-8 and a temperature of 20-30 \({}^{\circ}\)C, which aligns with the constant temperature of 24 \({}^{\circ}\)C in this experiment [31]. After 26 days of operation of the BSF reactor, the Cr(VI) removal rates of all three filter materials reached a stable state. Among them, the removal rates of volcanic stone (93.39 \(\pm\) 1.69%) and coconut shell activated carbon (90.60 \(\pm\) 2.08%) were significantly (_p_ < 0.05) higher than that of quartz sand (68.46 \(\pm\) 2.89%). Among them, the average removal rate of volcanic stone was the highest, 2.79 percentage points higher than that of coconut shell activated carbon. When the system reached a stable stage, volcanic rock outperformed coconut shell activated carbon in Cr(VI) removal. This might be attributed to the abundance of Ca, Fe, and magnesium in volcanic rock, which can promote the biological activity of anaerobic microorganisms (such as Bacillus and Pseudomonas), thereby enhancing Cr(VI) removal. Further exploration of volcanic rock properties revealed that its unique pore structure and rich mineral composition play a key role in the Cr(VI) removal process. These characteristics not only provide more attachment sites for microorganisms but also facilitate interactions between microorganisms and Cr(VI), accelerating the reduction and immobilization process of Cr(VI).