The main functions of microorganisms for alleviating heavy metal pollution can be categorized as: reducing intake, increasing excretion, and metabolic detoxification. The use of gene editing to knock out specific channel protein genes can produce microorganisms that are hyperaccumulators of specific heavy metals [32]. Strengthening the metabolism and detoxification ability of microorganisms can effectively improve their heavy metal adsorption abilities, and the stress caused by heavy metals can induce them to produce many functional proteins [33]. These proteins have strong binding ability for cadmium, copper, mercury, chromium, and other metals, and can transform water-soluble heavy metals into less toxic precipitated and organic-bound states, thereby reducing metal bioavailability in cells [34]. The A. niger isolated and purified in the present study has previously been used to remove heavy metals from sewage [35]. Recent studies show that chitosan or other extracellular polymeric substances (EPS) on the cell wall of A. niger can cooperate with various functional groups to complete the extracellular adsorption of heavy metal ions [36,37,38]. In this study, FTIR analysis showed that many reducing groups on the surface of A. niger were involved in the fixation of Cr6+, such as -C=C-, -C=0, -C-O-C, and -NH3. In order to explore the binding form of chromium on A. niger’s surface, we compared the differences between extraction with 8000 rpm centrifugation and filtration, and the results showed that the chromium content of centrifugal extraction was slightly lower than that of filtration extraction. Combined with XPS analysis, this shows that A. niger’s membrane system can fix Cr6+ with a relatively firm structure (surface complexation) and reduce it to Cr3+ in large quantities, except that a small amount of Chromium is attracted by ion exchange. In the subsequent supplementary experiment, the filtered A. niger bodies were broken to compare the amount of chromium fixed by the membrane system and absorbed by the cytoplasm. The results showed that the main way for A. niger to absorb chromium is through intracellular adsorption, and the surface adsorption amount is positively correlated with the biomass of A. niger; moreover, there was little difference in the fixation capacity per unit membrane area. There is also an upper limit for intracellular adsorption. When the intracellular chromium content is too high, A. niger can reduce the intake of chromium by closing the transport channel, which is also reflected in the kinetic analysis of this study. In this study, three kinetic models (including the intra-particle diffusion model) were used to elucidate the adsorption process of A. niger in solution and soil. The fitting results of the intra-particle diffusion model show that chromium could not diffuse through A. niger membranes and required endocytosis or active transport to enter the cell; in the early stage of adsorption, it was mainly the surface adsorption of membrane system, and the internal diffusion rate was very slow. In the middle stage of adsorption, the surface adsorption reached a dynamic balance and the internal diffusion effect was enhanced; additionally, detoxification by A. niger occurred through intracellular metabolism, and it stored chromium in the cells. At the late stage of adsorption, there was no internal diffusion effect, the absorption rate of chromium by A. niger was equal to the discharge rate or A. niger stopped absorbing chromium, and the adsorption effect reached a dynamic balance. Based on the other analysis, we found that the increase in Cr6+ concentration could improve the chromium adsorption rate of A. niger; however, the increase in the adsorption rate was limited by the combined action of the strong driving force of high concentration and binding sites on the surface of A. niger. These conclusions improve on the theory proposed by Chhikara [39].

In the red soil environment, a low dose of A. niger inoculation was more consistent with the intra-particle diffusion model. This is most likely because the complex soil environment can passivate chromium in addition to A. niger; therefore, when A. niger concentration was low, the adsorption process was not dominated by A. niger only. Compared with the microbiological agents purchased on the market, the application of the same amount of A. niger achieved significantly better results. Commercial microbial agents had a good effect within 7–10 d after inoculating, and then, the effect weakened. This phenomenon may be related to the difficulty in the colonization of exogenous microorganisms or the inadaptability of exogenous microorganisms to the acidic red soil environment. However, by increasing the biomass of A. niger, the passivation effect of hexavalent chromium in red soil was significantly improved, and the passivation rate was positively correlated with the addition amount.

As A. niger has a certain enrichment effect of chromium from the environment, in follow-up research, it could be transformed into a chromium hyperaccumulator through gene elimination and destruction of the channel protein synthesis gene that pumps out chromium. In addition, the results of the optimization study indicated that the optimal pH for chromium pollution remediation using A. niger was consistent with the main pH distribution of red soil, and that A. niger could drive the transformation of water-soluble and exchangeable states of chromium to precipitation and residue states while absorbing soil chromium. Moreover, its addition directly affected the rate of transformation of chromium state and greatly reduced the toxicity of chromium in red soil. In this study, the potential for indigenous A. niger to be used in treating heavy metal pollution in red soil was verified in many aspects. At the same time, this study proved that the activation of indigenous microorganisms was more effective than adding exogenous microorganisms, and the mechanism of chromium adsorption by A. niger was studied. Based on these conclusions, we can further put forward various improvement measures to improve the effectiveness of applications of indigenous A. niger.