**4.1. Interaction with microorganisms**

In the aquatic environment, certain bacteria are known to degrade non‐ionic surfactants into other phenolic compounds such as alkylphenolic compounds, most of which tend to be more toxic or harmful compared to the original compounds. For example, bacterial deg‐ radation of nonylphenol polyethoxylate surfactant results in the formation of nonylphenol. There is also evidence of microbial degradation of nonylphenoxyacetic acid into nitrophenol. Bacteria conversion of nonylphenol polyehoxylaes into other intermediate forms is also iden‐ tified to occur under certain specific environmental and chemical conditions. Nonylphenol polyehoxylaes are converted to nitrophenols under anaerobic conditions [28], and additional conversion of nitrophenol by iso‐substitution occurs under aerobic conditions [29]. Microbial interaction with 4‐chlorophenoxyacetic acid results in the production of 4‐chlorophenol. Pentachlorophenol degradation by bacteria produces tetrachlorocatechol which can undergo further degradation to form chlorinated catechols. Chlorocatechol was also identified to be the microbial degradation product of chlorobenzenes [30].

### **4.2. Interaction with inorganic compounds**

Ultraviolet radiation from the sun initiates a reaction between phenol and nitrite ions in the aquatic environment resulting in the formation of 2‐nitrophenol and 4‐nitrophenol [31]. In a similar manner, the interaction between hydroxyl radical and phenol in water produces 2‐ nitrophenol [32]. Conversion of phenol to nitrophenol also occurs with the availability of nitric ions. Photolysis of phenol in the presence of charge transfer complexes results in the formation of hydroquinone, while the formation of chlorophenol occurs through chlorination of aromatic compounds in water [33]. Some phenolic compounds also coordinate with metal cations of water enhancing their ionisation with the subsequent increase in their solubility in water [34].
