Coupling Microalgae Cultivation in Mine Wastewater with Soil Restoration in Coal Mine Spoil Heaps

Open-pit coal mining has created spoil dumps with poor soil structure and nutrient deficiency, challenging ecological restoration. This study developed a low-cost integrated remediation strategy combining microalgae, organic fertilizer, and cold- and drought-tolerant plants. First, local mine water was evaluated for microalgae cultivation. HS5 grew best in untreated mine water supplemented with partial BG11, a 0.1 g L−1 inoculation density, and 25–50 μmol m−2 s−1 light. Then, a 90-day field experiment was conducted, which included a control (CK), microalgae (M), organic fertilizer (F), and combined microalgae + organic fertilizer (MF), all sown with Dahurian wildrye and sainfoin. Compared with the CK, the M and F, both alone and combined, promoted plant growth and improved the soil properties, with the MF performing best. Compared with the CK, the MF significantly increased the total nitrogen content by 107.4%, soil organic matter by 89.5%, and available phosphorus by 52.7%, and raised the fresh and dry weights of both plants by 759.4–1084.2% and 660.6–891.7%, respectively. The MF also enriched the beneficial bacterial phyla, such as Bacillota and Bacteroidota, while reducing Acidobacteriota. These findings indicate that mine water-based microalgae cultivation is feasible, and that a microalgae–organic fertilizer–herbaceous plants system is a promising low-cost and sustainable strategy for restoring coal mine spoil dumps.