Y.Z., conceptualization, funding acquisition, investigation, data collection, project administration, resources, writing—original draft and editing; J.L., investigation, statistical analysis and writing, review and editing the manuscript; Z.Z., Y.L., Y.T., W.C., C.M., and F.C., writing, editing, and reviewing the manuscript. All authors have read and agreed to the published version of the manuscript.

Figure 1. Distribution of different Ni forms after the addition of exogenous Ni (0, 100, 200, 300, 600 mg/kg) to the mine soil.

Figure 1. Distribution of different Ni forms after the addition of exogenous Ni (0, 100, 200, 300, 600 mg/kg) to the mine soil.

Figure 2. Distribution of different Ni forms after the addition of citric acid (0, 1, 10 mmol/L) to the air-dried mine soil.

Figure 2. Distribution of different Ni forms after the addition of citric acid (0, 1, 10 mmol/L) to the air-dried mine soil.

Figure 3. Distribution of different Ni forms in Ni-100 soil after the addition of citric acid (0, 1, 10 mmol/L).

Figure 3. Distribution of different Ni forms in Ni-100 soil after the addition of citric acid (0, 1, 10 mmol/L).

Figure 4. Distribution of different Ni forms in Ni-200 soil after the addition of citric acid (0, 1, 10 mmol/L).

Figure 4. Distribution of different Ni forms in Ni-200 soil after the addition of citric acid (0, 1, 10 mmol/L).

Figure 5. Distribution of different Ni forms in Ni-300 soil after the addition of citric acid (0, 1, 10 mmol/L).

Figure 5. Distribution of different Ni forms in Ni-300 soil after the addition of citric acid (0, 1, 10 mmol/L).

Figure 6. Distribution of different Ni forms in Ni-600 soil after the addition of citric acid (0, 1, 10 mmol/L).

Figure 6. Distribution of different Ni forms in Ni-600 soil after the addition of citric acid (0, 1, 10 mmol/L).

Figure 7. Distribution of different Ni forms after the addition of malic acid (0, 1, 10 mmol/L) to the air-dried mine soil.

Figure 7. Distribution of different Ni forms after the addition of malic acid (0, 1, 10 mmol/L) to the air-dried mine soil.

Figure 8. Distribution of different Ni forms in Ni-100 soil after the addition of malic acid (0, 1, 10 mmol/L).

Figure 8. Distribution of different Ni forms in Ni-100 soil after the addition of malic acid (0, 1, 10 mmol/L).

Figure 9. Distribution of different Ni forms in Ni-200 soil after the addition of malic acid (0, 1, 10 mmol/L).

Figure 9. Distribution of different Ni forms in Ni-200 soil after the addition of malic acid (0, 1, 10 mmol/L).

Figure 10. Distribution of different Ni forms in Ni-300 soil after the addition of malic acid (0, 1, 10 mmol/L).

Figure 10. Distribution of different Ni forms in Ni-300 soil after the addition of malic acid (0, 1, 10 mmol/L).

Figure 11. Distribution of different Ni forms in Ni-600 soil after the addition of malic acid (0, 1, 10 mmol/L).

Figure 11. Distribution of different Ni forms in Ni-600 soil after the addition of malic acid (0, 1, 10 mmol/L).

Figure 12. Distribution of different Ni forms at different aging times (1, 3, 5, 7, 15, and 30 days) after the addition of citric acid to Ni-300 mg/kg contaminated soil.

Figure 12. Distribution of different Ni forms at different aging times (1, 3, 5, 7, 15, and 30 days) after the addition of citric acid to Ni-300 mg/kg contaminated soil.

Figure 13. Distribution of different Ni forms at different aging times (1, 3, 5, 7, 15, and 30 days) after the addition of malic acid to Ni-300 mg/kg contaminated soil.

Figure 13. Distribution of different Ni forms at different aging times (1, 3, 5, 7, 15, and 30 days) after the addition of malic acid to Ni-300 mg/kg contaminated soil.

Table 1. Concentration and type of organic acid added to Ni-contaminated reclaimed soil samples.

Table 1. Concentration and type of organic acid added to Ni-contaminated reclaimed soil samples.

Types of Organic AcidsMolecular FormulaMolecular WeightAdd Concentration (mmol/L)Citric acidC6H8O7192.140110Malic acidC4H6O5134.090110

Table 2. Measured concentrations of total nickel (all fractions) in spiked remediation soil samples and the range of relative deviation.

Table 2. Measured concentrations of total nickel (all fractions) in spiked remediation soil samples and the range of relative deviation.

Measurement ObjectSpiked Concentration of Ni-Contaminated Soil (mg/kg)Measured Mean Total Ni Concentration (mg/kg)Relative Deviation (%)A10095.311.36B200236.562.69C300301.660.55D600563.693.05

Table 3. Physicochemical properties of the reclaimed soil sample.

Table 3. Physicochemical properties of the reclaimed soil sample.

Capacity (g/cm3)pHOrganic Matter (g/kg)Available Potassium (mg/kg)Available Phosphorus (mg/kg)Available Nitrogen (mg/kg)1.337.84.13191.0310.4128.98