1. Context and Sample Data

The document presents results of water analysis for irrigation conducted by the Centro de Preservação Ambiental Tia Telinda, in Campos dos Goytacazes, RJ. The sample of interest, identified as INT 1 (Sample A), was collected from a clay pit in the Mineiros locality and exhibits the following chemical characteristics:

• pH: 2.7 (extremely acidic)
• Electrical Conductivity (C.E.): 7,561 dS/m (very high salinity)
• Sodium (Na): 21.35 mg/dm³
• Calcium (Ca): 21.63 mg/dm³
• Magnesium (Mg): 25.20 mg/dm³
• Sulfate (SO₄²⁻): 1980.00 mg/dm³ (exceptionally high concentration)
• Iron (Fe): 13.52 mg/dm³
• Manganese (Mn): 14.62 mg/dm³
• Chloride (Cl): 878.75 mg/dm³
• Total Solids: 4128.00 mg/dm³
• SAR (Sodium Adsorption Ratio): 4.41
• Carbonates (CO₃) and Bicarbonates (HCO₃): 0.00 mg/dm³ (absence of alkalinity)

The INT 1 sample shows high acidity (pH 2.7), elevated salinity, and significant concentrations of sulfate and metals such as iron and manganese. The absence of carbonates and bicarbonates indicates a natural inability to buffer, which explains the “extreme difficulty in accepting alkalization” mentioned.

2. Study Methodology

The study utilized phenolic compounds extracted from the bark of Red Aroeira (Schinus terebinthifolius), a native Brazilian species known for its chemical properties, such as tannins and phenolic compounds with coagulant and antioxidant potential. The process was described as follows:

• Extraction: 100 g of aroeira bark were used in 4 liters of water, subjected to pressurized hydrothermal extraction.
• Application: After extraction, 10 ml of the resulting solution were added to 1 liter of the INT 1 sample, along with 1 g of sodium bicarbonate (NaHCO₃) as an alkalizing agent.
• Observation:
  • Upon initial contact, the water changed from translucent to black instantly.
  • After 24 hours, there was an alteration in the solution’s state, possibly with precipitate formation.

3. Results and Interpretation

3.1. Initial Reaction (Black Coloration)

The instantaneous change to a black coloration suggests a rapid chemical reaction. The phenolic compounds from aroeira, such as tannins, are known to interact with metals, particularly iron (Fe²⁺ or Fe³⁺), forming dark-colored complexes. The high concentration of iron (13.52 mg/dm³) in the INT 1 sample likely reacted with the tannins, resulting in insoluble iron-tannin compounds responsible for the observed coloration. Sodium bicarbonate (NaHCO₃) was added to raise the pH, but the presence of iron sulfate (FeSO₄) and extreme acidity may have limited its initial effectiveness, favoring the formation of these dark complexes before any significant precipitation occurred.

3.2. After 24 Hours (Oxidation and Precipitation)

The change observed after 24 hours, possibly with lodo formation, can be attributed to the oxidation of iron sulfate (FeSO₄) present in the water. In an environment with low pH and high sulfate concentration (1980 mg/dm³), ferrous iron (Fe²⁺) can oxidize to ferric iron (Fe³⁺) in the presence of oxygen, forming iron hydroxides (such as Fe(OH)₃), which precipitate as a reddish or brownish lodo. The interaction with phenolic compounds may have catalyzed or stabilized this process, promoting coagulation. Over time, sodium bicarbonate may have begun to partially neutralize the acidity, raising the pH and favoring the precipitation of heavy metals such as iron and manganese.

4. Discussion

4.1. Effectiveness of Aroeira Phenolic Compounds

The tannins from aroeira demonstrated potential as a natural coagulant, removing iron and possibly other metals from the aqueous solution through complex formation and subsequent precipitation. Compared to synthetic coagulants such as aluminum sulfate (Al₂(SO₄)₃) or ferric chloride (FeCl₃), phenolic compounds offer a sustainable alternative, especially since they use commercial production waste, preserving wild plants.

4.2. Limitations of Alkalization

The difficulty in correcting the pH in a natural environment is confirmed by the absence of carbonates/bicarbonates and the high concentration of sulfates, which maintain elevated acidity. Sodium bicarbonate alone was not sufficient to completely neutralize the sample.

4.3. Lodo Toxicity

The lodo formed contains iron, manganese, and potentially other metals present in the original water (such as zinc and copper in smaller amounts). The toxicity of this residue depends on the concentration of these elements and their bioavailability. Phenolic compounds may reduce the solubility of metals, making the lodo less toxic than residues generated by synthetic coagulants, but this requires chemical analysis and leaching tests (e.g., TCLP – Toxicity Characteristic Leaching Procedure).

5. Conclusions and Recommendations

• Effectiveness: The phenolic compounds from Red Aroeira proved promising as natural coagulants, removing iron and possibly other metals from acidic water. The combination with sodium bicarbonate aided precipitation but did not fully correct the acidity.
• Sustainability: Using commercial aroeira waste is an eco-friendly approach, reducing environmental impact compared to traditional chemical coagulants.
• Toxicity: To determine if the lodo is safe for disposal or reuse (e.g., as fertilizer), it is essential to conduct toxicity tests and detailed chemical analysis of the precipitate.
• Next Steps:
  1. Increase the dosage of bicarbonate or test other alkalizing agents (e.g., Ca(OH)₂) to neutralize the pH.
  2. Quantify metal removal after treatment.
  3. Evaluate lodo toxicity with standardized assays.

6. Perspectives

This study paves the way for developing more sustainable and potentially less environmentally harmful alternatives for treating highly acidic waters. Using aroeira byproducts not only adds value to commercial production waste but can also contribute to water management practices that preserve natural resources and local biodiversity.

In summary, the initial results suggest that phenolic compounds from aroeira exhibit promising reactive behavior in treating acidic waters with high iron content, although toxicity assessments and comparisons with other methods are critical steps to validate their safe and effective applicability.