Adsorption Characteristics of Indigenous Chromium-Resistant Aspergillus niger Strain Isolated from Red Soil for Remediation of Toxic Chromium in Red Soil Environments

The microbial treatment of soil has great potential to reduce chromium pollution. Here, an indigenous chromium-resistant Aspergillus niger strain (A1) was isolated and screened from heavily chromium-contaminated red soil in Yunnan Province, China using a traditional isolation method and a selective culture experiment. The molecular identification of A1 was achieved using 18S rRNA sequencing. The tolerance of the strain to toxic chromium was evaluated through pure laboratory culture. The adsorption effect and mechanism of A1 on chromium in red soil were further studied. The study concluded that A1 exhibited strong activity with exposure to 500 mg·L−1 Cr6+. Chromium adsorption by A. niger occurred mainly through intracellular metabolism, surface complexations with EPS, and chemical reduction with -C=C-, -OXuH, NH2, and -C=0. The optimized results showed that A1 had the best Cr6+ removal effect at pH 4, 40 °C, and a 60 h culture time. Compared with the inoculating of exogenous microbial agents, after inoculating A1 into the chromium-contaminated red soil, Cr6+ content was significantly reduced, and the high-toxicity chromium state (water-soluble and exchange states) decreased, whereas the low-toxicity chromium state (precipitation and residue states) increased. The results of red soil ITS also showed that the inoculation of indigenous microorganisms can better colonize the red soil. This study proves the feasibility of the application of indigenous A. niger to address red soil chromium pollution and provides a new idea and theoretical support for red soil remediation.