*3.2.2. Graphene materials and graphene‐based materials*

Graphene materials have the capacity to transfer the charges rapidly which is very impor‐ tant to reduce the charge recombination. Besides this, the photocatalytic activity of graphene oxide, a functional form of graphene, has been proved [76]. Therefore, a review of the inves‐ tigations where graphene materials have been used individually and in combination with others compounds for the removal of phenolic compounds is presented.

Bustos‐Ramirez et al. [77] investigated the removal of phenol by photocatalysis using as pho‐ tocatalyst GO synthesized under different conditions. The time of oxidation (2, 4 and 6 h) and the degassing units (55 and 65) were modified. The samples were labeled as GEO‐2‐55, GEO‐2‐65, GEO‐4‐55, GEO‐4‐65, GEO‐6‐55 and GEO‐6‐65. The experiment was carried out in a batch photoreactor containing 100 mL of initial concentration of 100 mg/L. The time of reac‐ tion was 2 h and as radiation source an UV lamp of 254 nm was used. The band gap obtained by all samples indicated that these materials could act as photocatalysts. The best results of phenol removal were found with the sample GO‐2‐55 (38.62%), followed by GO‐6‐55 (14.96%) and GO‐4‐55 (12.29%). **Figure 5** shows the absorption spectra of phenol before and after of the photocatalysis process using the GO‐2‐55 sample as photocatalyst. The degassing units and the oxygen functional groups played an important role in the preparation of GO and its photocatalytic activity, respectively. The obtained results indicated that GO under specific synthesis conditions is a very viable material for its use in photocatalytic processes for the contaminants degradation.

A better photocatalytic activity of GO was found on the degradation of 4‐clorophenol (4‐CP) [78]. The photocatalytic experiments were carried out using 30 mL of phenol solution with initial concentration of 30 mg/L, pH 7 and GO dosage of 0.8 g/L. As irradiation source was

**Figure 5.** Phenol absorption spectra at initial and after of 2 h of photocatalytic reaction (reprinted with permission of Karina Bustos‐Ramirez et al. [77]. Copyright © 2015, used under the Creative Commons Attribution License).

(SiO2

Photocatalytic‐UV‐C irradiation

Solar light photocatalytic

Heterogeneous photocatalytic‐UV‐laser

irradiation

degradation

TiO2

) and zeolite (ZSM‐5). All materials TiO<sup>2</sup>

BiPO4

360 Phenolic Compounds - Natural Sources, Importance and Applications

support followed by ZSM‐5 and SiO<sup>2</sup>

contaminants degradation.

formance on the 4‐chlorophenol degradation than TiO<sup>2</sup>

Ag‐core TiO2

on phenols degradation in water are shown in **Table 1**.

*3.2.2. Graphene materials and graphene‐based materials*

‐supported reached a better photocatalytic per‐

Phenol [73]

Phenol [74]

Phenol [75]

. The maximum degradation of 4‐chlorophenol using

‐AC as photocatalyst was 89.7%. Others photocatalytic materials that have been studied

**Proposal system Type of photocatalytic material Degraded component References**

Ag/ZnO Nitrophenol [72]

One photocatalytic step Bismute vanadate, BiV4 Phenol [71]

nanoparticles under

 photocatalytic activity degradation under UV‐C irradiation

ZnO nanoparticles coupled under

UV‐laser irradiation

solar light irradiation

**Table 1.** Photocatalytic materials used on the removal of phenolic compounds.

Graphene materials have the capacity to transfer the charges rapidly which is very impor‐ tant to reduce the charge recombination. Besides this, the photocatalytic activity of graphene oxide, a functional form of graphene, has been proved [76]. Therefore, a review of the inves‐ tigations where graphene materials have been used individually and in combination with

Bustos‐Ramirez et al. [77] investigated the removal of phenol by photocatalysis using as pho‐ tocatalyst GO synthesized under different conditions. The time of oxidation (2, 4 and 6 h) and the degassing units (55 and 65) were modified. The samples were labeled as GEO‐2‐55, GEO‐2‐65, GEO‐4‐55, GEO‐4‐65, GEO‐6‐55 and GEO‐6‐65. The experiment was carried out in a batch photoreactor containing 100 mL of initial concentration of 100 mg/L. The time of reac‐ tion was 2 h and as radiation source an UV lamp of 254 nm was used. The band gap obtained by all samples indicated that these materials could act as photocatalysts. The best results of phenol removal were found with the sample GO‐2‐55 (38.62%), followed by GO‐6‐55 (14.96%) and GO‐4‐55 (12.29%). **Figure 5** shows the absorption spectra of phenol before and after of the photocatalysis process using the GO‐2‐55 sample as photocatalyst. The degassing units and the oxygen functional groups played an important role in the preparation of GO and its photocatalytic activity, respectively. The obtained results indicated that GO under specific synthesis conditions is a very viable material for its use in photocatalytic processes for the

A better photocatalytic activity of GO was found on the degradation of 4‐clorophenol (4‐CP) [78]. The photocatalytic experiments were carried out using 30 mL of phenol solution with initial concentration of 30 mg/L, pH 7 and GO dosage of 0.8 g/L. As irradiation source was

others compounds for the removal of phenolic compounds is presented.

alone. AC was found to be the best

used an UV lamp (Pencil UV lamp, 254 nm and 5.5 W), which was introduced in the phenol solution. The photocatalytic results indicated that about of 80% of phenol was removed of the solution in a time of 100 min. About of 50% of removal was obtained in the first 20 min. The results of the chemical oxygen demand (COD) tests indicated that 97% of the organic matter was removed (**Figure 6**). Aromatic compounds and carboxylic acids are the main by‐products generated in this photocatalytic process. The high efficiency of GO on the 4‐CP degradation indicated that graphene materials have an important future in the photocatalysis area.

On the other hand, some investigations have studied the degradation of phenolic compounds employing composites of graphene and semiconductors particles. Some composites of gra‐ phene/TiO2 have been synthesized by different routes for the degradation of phenol from water [79–81]. In all cases, the combination of graphene materials and TiO2 particles improve the performance obtained with only TiO2 . This was attributed to an increase in the adsorp‐ tion of phenol molecules, a better and more efficient charge separation and the improvement light absorption. The degradation of phenol was so well studied using other composites of graphene‐based materials with good results [82–86].

Other phenolic compound that has been investigated was Bisphenol A. Wang et al. [87] syn‐ thesized a GO/ Ag<sup>3</sup> PO<sup>4</sup> composite and proved its efficiency on the BA degradation. The deg‐ radation of BA was carried out using 75 mL of a solution of BA with initial concentration of 20 mg/L and 75 mg of catalyst. A 300 W Xe lamp with a 400 nm cutoff filter was used as irradiation source. The results indicated that the GO/Ag3 PO<sup>4</sup> (6 wt%) composite improved the degradation of BA with respect to the pure Ag<sup>3</sup> PO<sup>4</sup> . This enhancing of the photocatalytic activity of GO/Ag3 PO<sup>4</sup> was attributed to the presence of GO, which contributed to the sepa‐ ration electron‐hole pairs in the composite. Similar results were found by Chen [88] on the

**Figure 6.** COD removal percentage obtained at the end of photolysis and photocatalysis tests using as catalysts (graphite oxide) GrO and GO on the 4‐CP degradation. Adapted from Bustos‐Ramírez et al. [78]; BioMed Central. 2015.

2,4‐dichlorophenol degradation using as photocatalyst a GO/Ag<sup>3</sup> PO<sup>4</sup> (5 wt%) composite. It is important to note that in both investigations was used visible light which is one of the most important challenges in the photocatalysis process.

In general, the revised investigations revealed that the combination of graphene materials with different semiconductors particles improve the degradation efficiency of the different phenolic compounds from water with respect to the individual particles. Besides, the graphene oxide showed an important photocatalytic activity capable of degrading the phenolic compounds.
