**6.5. Adsorption**

**6.4. Biological method**

430 Phenolic Compounds - Natural Sources, Importance and Applications

Biological method of phenolic compound removal from wastewater is subdivided into micro‐ bial and enzymatic methods. The microbial method involves the deployment of bacteria, yeast and fungi in breaking down the phenolics into harmless products such as carbon dioxide and water. This method of phenolics removal is feasible as a result of the fact that some microorgan‐ isms are known to depend on aromatic compounds, including phenolics, as their source of car‐ bon or nutrient [63, 64]. It has the advantage of a comparatively low operational cost. Microbial removal of phenolic compounds occurs through either aerobic or anaerobic processes and begins with hydroxylation (introduction of hydroxyl groups) of the aromatic ring [65]. Hydroxylation through aerobic degradation involves two steps with catechol being the end product [66]:

**(1)** Reduction of one of the molecular oxygen to water under the influence of a hydrogen donor (reduced pyrimidine nucleotide), and devouring of the other oxygen atom.

**(2)** The second step of the hydroxylation process occurs in the presence of dioxygenase

Cleavage of the catechol aromatic rings then passes through various stages with specific enzymes, based on the type of microorganisms, resulting in the conversion of the phenolic compounds to compounds such as carbon dioxide and water [67]. Anaerobic degradation

This process is believed to be initiated by carboxylation of phenol to 4‐hydroxybenzoate [68].

In general, the aerobic process is known to be better suited for the degradation of phenolics with minimal substituents consisting of halogens. On the other hand, the anaerobic process is mostly appropriate for reduction of chlorinated phenolic compounds [69]. The anaerobic system produces methane in addition to carbon dioxide and water. A major advantage of the anaerobic system of degradation is the absence of aeration cost, recovery of methane and

Kukadiya et al. [71] studied the effectiveness of using a moving bed biofilm reactor for phe‐ nolic compounds removal from wastewater. The laboratory scale model moving bed biofilm reactor was observed to be effective against the removal of phenol with about 98% efficiency. In their experiment, Sinha et al. [72] studied the p‐chlorophenol and phenol microbial deg‐ radation as a single and mixed substrates by using *Rhodococcus* sp. RSP8 bacteria strain. The experiment was performed with a liquid mineral salt medium in a shake flask experiment at a neutral pH and a temperature of 37 °C. The two compounds (p‐chlorophenol and phenol) served as the main source of carbon and energy for the cells and were consumed completely as individual solutions by the cells. The two pollutants, however, repressed each other's deg‐

The enzymatic method of degradation, however, employs enzymes (biological catalysts). Enzymes can be used effectively to selectively eliminate pollutants in water since they catalyse specific reactions under modest temperature, pH and ionic strengths [73]. In addi‐ tion, the enzymatic reaction is known to occur at much faster rates compared to other types of reactions [74]. As an advantage over the microbial system of pollutant degradation, the

, or light are present.

enzyme with the subsequent formation of catechols.

minimum excess biomass generation [70].

radation by the cells in the mixed substrate experiment.

occurs whenever oxidising agents such as sulphates, nitrates and CO2

Adsorption is considered as one of the appropriate techniques for removal of phenolics from water because the technique is easy to design and operate. The technique produces no toxic wastes. The spent sorbent can serve as a source fuel to produce power [81]. Adsorption process involves the accumulation of the pollutant on the adsorbent's surface (usually solid material). An appropriate adsorbent must be porous with large surface area, possess high hydrophobicity and have the ability to selectively accumulate the pollutant from water onto its surface. Efficiency of the adsorption process is governed by [82]:


Reference [82] found phenol adsorption process to be solely dependent on the initial pollut‐ ant concentration and speciation, which in turn depends on pH of the solution. Adsorption of pollutants from water is believed to be based on the following steps [83]:


Various researchers have studied phenol adsorption from polluted water with different types of adsorbents. Phenol adsorption efficiency of different adsorbents including bagasse ash, activated carbon and charcoal from wastewater was studied by [84]. The adsorption efficiency was assessed based on the influence of pH, concentration of EDTA, anions and adsorbent dose. Their result showed 98, 90 and 90% phenol removal efficiencies by activated carbon, wood charcoal and bagasse ash systems, respectively. Removal efficiency was observed to increase with a decrease in the pH of the system. Effects of EDTA and nitrate ion content of the solution were identified as the factors that influenced the adsorption process. Chloride ion, on the other hand, exerted a significant adverse effect on the efficiency of bagasse ash system. Film diffusion was noted to control the adsorption efficiencies of all the adsorbents used. Similarly, the use of sugarcane bagasse‐based activated carbons for effective phenol adsorption from aqueous medium was assessed by Akl et al. [85]. The result of the study pro‐ posed sugarcane bagasse‐based activated carbon (SCBAC) as a viable adsorbent for phenol elimination from water. The pollutant eradication process depended solely on its concentra‐ tion, solution pH and temperature.
