Environmental Pollution and Degradation of Metribuzin

Metribuzin is an internally selective triazinone herbicide developed by Bayer AG in Germany in 1971. It controls weeds by disrupting photosynthesis and is suitable for pre-emergence application in crops such as soybeans, potatoes, sugarcane, and coffee to manage annual broadleaf and grass weeds. While generally ineffective against perennial grasses, Metribuzin also provides control over algae.

Figure 1: The Picture of Metribuzin.

General Description

Metribuzin is a cost-effective non-selective herbicide belonging to the chemical class of triazinones. It inhibits plant DNA synthesis and targets photosystem II, ultimately suppressing photosynthesis. Metribuzin provides effective control of major annual grasses and broadleaf weeds. Metribuzin is a selective triazinone herbicide applied in both pre- and post-emergence agricultural practices for the control of weeds in a variety of crops including soybeans, potatoes, tomatoes, and sugar cane, which functions by inhibiting the photosynthetic process through the specific disruption of the electron transport chain within photosystem II, and its extensive use has resulted in its classification as a persistent environmental contaminant frequently detected in groundwater sources.

Properties

Metribuzin is a white crystalline solid that is slightly soluble in water but readily soluble in organic solvents such as methanol and toluene. It is stable in dilute acids and strong alkalis. This herbicide exhibits low toxicity to humans, livestock, and fish, with an oral LD50 in rats of 1100–2300 mg/kg. It has the potential to leach into groundwater. Metribuzin is absorbed by the roots, stems, and leaves of plants and is translocated throughout the plant. Its primary mode of action involves the inhibition of the photosystem II water-plastoquinone oxidoreductase enzyme in susceptible plants, which disrupts photosynthesis and ultimately leads to the yellowing and withering of the weeds.

Synthesis

Metribuzin is produced by reacting one mole of 4-amino-6-tert-butyl-3-mercapto-(1,2,4)triazin-5(4H)one and half a mole of dimethyl sulfonate which react at 57°C in presence of sulfuric acid media about 7 hours and transfer methyl (CH3) from triazine to metribuzin and product formed 1 mole of metribuzin and half mole of sulfuric acid and later neutralized with soda ash and then purified.

Environmental Pollution

Metribuzin can accumulate in soil and water, causing harm to both plants and animals. The herbicide exhibits highly variable toxicity: short-term exposure may lead to acute poisoning, resulting in symptoms such as respiratory difficulties and drowsiness, while higher exposure levels can trigger abdominal pain, fatigue, and central nervous system depression. Long-term or repeated intense exposure may cause alterations in liver enzymes, kidney dysfunction, reproductive abnormalities, endocrine disruption, and an imbalance in the oxidant/antioxidant system. Human exposure to metribuzin occurs primarily through inhalation and ingestion, most commonly in agricultural settings. Occupational exposure among farmers varies significantly and is highly complex, depending on farm conditions. However, studies assessing exposure in the general population remain very limited. Research has confirmed that even low-dose exposure poses toxic risks. The hazards are not limited to occupationally exposed individuals; the general public may also be affected through the consumption of agricultural products treated with this pesticide, constituting a tangible public health concern. [1]

Soil Degradation

The persistence of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)-one] in several soils from the lower alluvial floodplain of the Mississippi River was studied in the greenhouse and laboratory using bioassay and gas chromatographic methods of residue detection. Bioassay results indicated that metribuzin phytotoxicity was lost in several soil types within a few weeks of application. Longevity of phytotoxicity depended upon soil type. However soil type had little effect on the rate of metribuzin degradation measured chemically. Gas chromatographic techniques indicated that metribuzin degradation followed first-order kinetics with half-life values ranging from 17 to 28 days in six soils under greenhouse conditions. Incubation studies indicated that metribuzin was degraded more rapidly in nonautoclaved field soil and in soil enriched with glucose than in soil that had been air dry for 1 yr or had been autoclaved. Degradation was more rapid at 30 C than at 20 C. Microbiological activity apparently is important in the degradation of metribuzin in soil. [2]

Reference

[1] Samir D, The effect of herbicide metribuzin on environment and human: A Systematic Review[J]. Pharmaceutical and Biosciences Journal, 2020: 10-15.

[2] Savage K E. Metribuzin persistence in soil [J]. Weed Science, 1977, 25: 55-59.

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