The results you're observing can be explained by several biochemical processes and physiological responses of *Pistia stratiotes* (water lettuce) during phytoremediation:
1. **Initial TDS Increase (Days 1-7)**:
- **Leaching of Ions**: When water lettuce is first introduced to wastewater, it can initially release ions or dissolved organic compounds from its tissues, which can temporarily increase TDS. This is common when plants adapt to a new environment, as they may release exudates, ions, and metabolites as part of a stress response.
- **Microbial Activity**: Water lettuce roots support a variety of microbes that help with nutrient breakdown and decomposition. This microbial activity can release certain ions or breakdown products, contributing to TDS.
- **Decomposition of Existing Organic Matter**: As the plants begin to uptake nutrients and remove organic contaminants (evident in reduced COD and turbidity), microbial processes may convert complex organic materials into simpler, soluble ions, leading to a rise in TDS.
2. **TDS Decrease (Days 8-10)**:
- **Plant Uptake of Dissolved Ions**: Once the initial adjustment period has passed, water lettuce can begin effectively absorbing dissolved ions, including some components contributing to TDS. Commonly absorbed ions include nitrates, phosphates, and certain metals. The slight decline in TDS aligns with the phase where the plant has adapted to the wastewater environment and begins actively reducing dissolved contaminants.
- **Enhanced Root Zone Microbial Activity**: As the water lettuce adapts, it may enhance root zone microbial populations, which in turn can immobilize certain ions or convert them into forms that precipitate out of solution, reducing TDS.
- **Reduction of Other Contaminants**: As TDS is often indirectly influenced by the levels of organic and inorganic compounds, the continued reduction in COD, nitrate, phosphate, TSS, and turbidity may indicate that the overall pollutant load is declining, leading to a gradual decrease in TDS as well.
3. **Steady-State or Equilibrium Phase**:
- If the experiment continues, you might observe a stable or reduced TDS over time as the plant and microbial communities reach a steady-state, balancing the removal of dissolved substances with any residual release of ions.
The observed pattern suggests that water lettuce initially undergoes an adaptation phase, after which it begins to actively reduce both dissolved and particulate contaminants in the wastewater. This aligns well with phytoremediation mechanisms, where a lag period is typical before substantial pollutant removal begins.
