Hebei Messi Biology Co., Ltd. stated that magnesium oxide is currently one of the most potential curing/stabilizing repair materials. Using magnesia itself as a gelling agent, or magnesia-based green cement developed on the basis of magnesia, have shown excellent remediation effects in the stabilization and restoration of heavy metals in soil. Compared with the traditional curing/stabilizing repair material Portland cement (production temperature up to 1450°C), the production process of magnesia consumes less energy (usually 500-800°C); Magnesite is obtained by calcination, and can also be extracted from seawater and brine; the durability of magnesium oxide is also better than that of Portland cement.
Soil lead pollution exists widely in the world. Lead has significant harm to human health, especially children’s intellectual development. Solidification/stabilization technology is currently the most widely used remediation technology for lead-contaminated soil, but the biggest challenge of solidification/stabilization technology is how to ensure the long-term effectiveness of remediation; at the same time, the onset time of remediation in the short term is also worthy of attention.
The activity of magnesia is controlled by its specific surface area and crystallinity. For magnesia prepared by calcination, its specific surface area and crystallinity are controlled by calcination temperature. Therefore, there is a significant correlation between the activity of magnesium oxide and the calcination temperature (Figure 1).
Figure 1 There is a significant correlation between the activity of magnesia and the calcination production temperature (the unit of the ordinate is seconds, the shorter the time, the higher the activity of magnesia)
Specifically, the lower the calcination temperature, the higher the activity. The activity of magnesium oxide determines its reaction rate with soil heavy metals (ie, the higher the activity, the faster the reaction), thus determining the time effect of its stabilization and remediation of soil heavy metals from the macroscopic scale. Therefore, the hypotheses put forward in this study are: 1) On a short-term scale (such as 0-49 days), the effect of magnesium oxide on lead-contaminated soil remediation is controlled by its activity, and the higher the activity of magnesium oxide, the faster the remediation effect will be. 2) On a long-term scale (such as 0-100 years), the repair effect of magnesium oxide is also controlled by its activity. The higher the activity of magnesium oxide, the worse its long-term effectiveness, because it is more likely to react with acid rain and fail, while Magnesium oxide with lower activity, although slow to act in the short term, is more resistant to acid rain and thus has better long-term effectiveness.
Based on the above assumptions, this study aimed at a lead-contaminated industrial site soil (4280 mg/kg), using four kinds of magnesium oxide prepared from the same raw material magnesite but with different activities for stabilization and remediation. The study found that the four types of magnesium oxide can reduce the toxic leaching concentration of lead in soil from 1.8 times exceeding the standard to below the standard within one day, and complete the restoration goal (Figure 2). Within 1-49 days after repair, the repair remains effective, while the activity of magnesium oxide has little effect on its short-term repair effect.
Figure 2 Changes in soil lead toxicity leaching concentrations in the short term (1-49 days) after remediation (the five groups of samples represent the original contaminated soil and the remediation soil with magnesium oxide activity from high to low)
Subsequently, using the self-developed quantitative artificial accelerated aging method, the impact of 25-100 years of rain erosion in the field on the stability of soil lead after remediation was simulated in the laboratory. It was found that within 50 years after remediation, the toxic leaching concentration of lead in soil reached the standard; in the 75th year, the leaching concentration of lead in the soil after remediation with the lowest activity magnesium oxide exceeded the standard, and the rest reached the standard; in the 100th year, only the soil after remediation with the highest activity magnesium oxide The lead toxicity leaching concentration reached the standard, and the rest exceeded the standard (Figure 3).
Figure 3 Changes of soil lead toxicity leaching concentration in the long-term scale (25-100 years) after remediation (the five groups of samples represent the original contaminated soil and the remediation soil with MgO activity from high to low)
Further continuous extraction experiment analysis shows that after 100 years of simulated rain erosion, there is basically no stable lead in the original lead-contaminated soil, while the stable lead content in the soil repaired by the most active magnesium oxide is significantly higher than Soil after remediation with less active MgO (Fig. 4).
Figure 4 The form and composition of soil lead after 100 years of simulated rain erosion (the five groups of samples represent the original polluted soil and the remediation soil with magnesium oxide activity from high to low)
Therefore, this study overturned the preset experimental assumptions and found that in the short-term after repair, the activity of magnesium oxide has little effect on the repair effect, while on the simulated long-term scale, the long-term repair effect of magnesium oxide with higher activity The more effective it is. The main reason is that magnesium oxide with higher activity hydrates faster and generates more hydration intermediates containing carbonate, which improves the neutralization and buffering capacity of the entire system against acid, and is more conducive to resisting acid rain erosion. Overall, this study found that magnesium oxide is a low-carbon and highly efficient material for soil lead stabilization. It works very quickly (within 1 day), and by adjusting its activity, it can maintain the restoration effectiveness for more than 100 years. .