A study of 4-chlorophenol continuous adsorption on nano graphene oxide column: model comparison and breakthrough behaviors

Removal of 4-chlorophenol in a continuous ﬁ xed-bed column was investigated by using nano graphene oxide (NGO) adsorbent in this study. The adsorbent (NGO) was characterized by X-ray diffraction (XRD) analysis and scanning electron microscopy analysis. Variables in the adsorption process were bed depths (5, 10 and 15 cm), ﬂ ow rate (1, 2 and 4 mL/min), in ﬂ uent 4-chlorophenol concentrations (5, 10, 15, 20 and 30 mg/L) and in ﬂ uent solution pH (6 – 7). The enhancement of adsorption is favored by decreasing 4-chlorophenol concentration and ﬂ ow rate and increasing bed depth. Indeed, the best result obtained in this study was 145.2 mg/g of adsorption capacity under 4-chlorophenol concentration of 5 mg/L at a ﬂ ow rate of 1 mL/min toward a bed depth of 15 cm. The results of the study showed that the ideal 4-chlorophenol adsorption followed well the Thomas and Yoon – Nelson models for predicting breakthrough behavior at different ﬂ ow rates and bed depths. Finally, increasing ﬂ ow rate, decreasing bed depth and increasing in ﬂ uent 4-chlorophenol concentration resulted in the breakthrough time and exhaustion time decreasing.

. So, in order to protect the health of people and the environment and ensure economical use of available water resources, changes need to be made to reduce water pollution (Hamdaoui & Naffrechoux ).
Responsible agencies, such as the EPA, recommended a maximum contaminant level of phenol in water of 0.3 mg/L and for other halogenated phenols, such as pentachlorophenol, 0.001 mg/L (Johnson et al. ). Thus, the achievement of maximum concentration levels of phenols has become a major environmental problem. In the literature, different techniques like chemical oxidation, electrocoagulation, solvent extraction and membrane separation have been developed for removal of 4-chlorophenol from wastewater (Monsalvo et al. ). These techniques are expensive but among physicochemical processes, adsorption is a well-established and effective technique for the removal of organic matter from water, wastewater, etc. (Akar et al. ). Nano material like graphene oxide can be effectively employed in water and wastewater treatment. In fact, it is able to retain organic matters, which are resistant to physicochemical and biological treatments, and removes a large proportion of pollutants. Furthermore, nano materials exhibit high adsorption capacity, rapid uptake and selectivity as well as being inexpensive (Nishiyama et al. ). Graphene oxides for adsorption of organic compounds have been investigated by researchers but studies related to the granular form of nano powder are unheard of. So, the role of graphene oxide in 4-chlorophenol adsorption is investigated in the present study. The results of continuous systems are important for obtaining basic engineering data, as batch studies of adsorption with undetermined factors may not provide accurate scale-up information concerning the column operation system (Benefield et al. ).
The objectives were to: (1) perform batch and continues studies to examine 4-chlorophenol adsorption using nano graphene oxide (NGO) (effect of concentration, flow rate, time and bed depth) and (2) perform modeling studies to investigate the breakthrough behavior and exhaustion time of granular NGO and analyze using Adams-Bohart, Thomas and Yoon-Nelson models.

Characterization of adsorbent
Scanning electron microscopy (SEM) photomicrography of the granular NGO was carried out using an electronic microscope (Philips XI-30 ESEM-FEG Company, USA). X-ray diffraction (XRD) analysis on the adsorbent was carried out by means of a Philips PW1710/00 (Ireland) model.

Graphene oxide granules preparation
The sorbent was prepared at a concentration of 20 wt% and converted to granulated form in order to maintain the nano characteristics. A total of 2.5 g of the NGO powder (Nanosav Company, Iran) was mixed with 1,000 mL of water using a mechanical stirrer at 1,000 rpm for 2 hours. Due to the nano size of the graphene oxide powder, the particle tended to agglomerate, especially when dispersed in water. Next, a measured amount of sodium alginate powder was added to the graphene oxide powder solution to obtain 1% (w/v) alginate solution. The suspension was stirred for 4-6 hours in order to ensure dispersion of the sodium alginate. Then 100 mL of 1 molar chloride calcium, CaCl 2 (Merck, Germany) was prepared and set on a mechanical stirrer at 100 rpm. The suspension (mixture of nano powder and sodium alginate) was added to chloride calcium solution by using a dropping-tube to form granules (Smirnova & Arlt ). Granulation occurred due to ion exchange of calcium to sodium. For complete ion exchange, granules were kept in the chloride calcium solution for 2 hours. After this time, granules were washed with distilled water and were dried in an oven (65-75 W C). After drying, granules were kept in a container.

Batch and continuous adsorption system studies
Batch system experiments were carried out at 25 W C using  (1): where q m is the amount of metal ions adsorbed per unit weight of adsorbents (mg/g), Q is the flow rate (mL/min) and w is the dry weight of the adsorbent packed in the column (g), C 0 is the initial influent concentration (mg/mL), t e is the time to adsorbent exhaustion (min), t b is the time to constituent breakthrough (min), f(t) is the function of the effluent curves obtained from column testing, t e À Ð te t b f (t) d t is the area of the breakthrough curve under exhaustion conditions, which can be estimated through integration.

Analysis procedure
The concentration of 4-chlorophenol at 500 nm in a UV-vis- The equation of breakthrough curve models and the parameters are given in Table 1.

Adsorbent characteristics
In this study, NGO granules were characterized by XRD, see Batch experiment description  Adams-Bohart    means that the rate of diffusion after the early stages of the adsorption increased (Mohan & Singh ). Generally, after NGO granule synthesis, the highly crystal structure was altered, self-aggregation of graphene layers was evidently relieved, and more single to few layer graphene nanosheets were created with folds and wrinkles meaning that the micro pore volume and specific surface area of the NGO granules increased relative to before granulation.
So, due to the more porous surface of the graphene and strong electron charge transfer, the adsorption rate was gradually promoted.

). Comparison between models presented in this study
with consideration of R 2 values, indicated both the Thomas and Yoon-Nelson models can be used to predict 4-chlorophenol adsorption performance.

Modeling of flow rate based breakthrough equations
The results given in Figure 4