Adsorption of Nitrophenols (NPhs) using N-Doped Carbonaceous Material

Summary

Nitrophenols (Nphs), a chemical used for synthesizing in pesticides, insecticides, herbicides and other industries,is a major source of water pollution. Due to population growth, the usage of these products are increasing exponentially and along with it water pollution related to NPhs. EPA reported 4.9 billion pounds of pesticide usage/year in the USA alone. Agro products used for farming, undergo hydrolysis and biodegradation to produce NPhs which are directly discharged into waste water and natural water resources. NPhs and their derivatives are highly toxic and has been classified as a priority pollutant by EPA. Current adsorbents used for NPhs remediation are expensive, inefficient, and unsustainable. The goal of this project was to find a suitable adsorbent using naturally abundant environmentally benign material to remove NPhs from water in an efficient, inexpensive and sustainable way. To achieve this, nitrogen doped carbonaceous material containing titanium dioxide (N-C-TiO2) was synthesized by the pyrolysis of microcrystalline cellulose (MCC), dopamine and TiO2 at 500 °C. NPh adsorption on the N-C-TiO2 adsorbent surface was then studied by varying pH, initial concentration of NPh, and adsorbent dose. After 4 hour of sonication 80% 4-NPh adsorption was achieved using N-C-TiO2 at pH 3.0. 4-NPh adsorption best fitted to the Langmuir isotherm plot with R2 value of 0.9981 and absorption capacity of this adsorbent is 52.91 mg·g-1. Adsorbent reusability was studied successfully for five cycles. N-C-TiO2, with absorption capacity of 52.91 mg·g-1 and being real cheap, proved to be a real good solution comparing to the current alternatives.

                                                   

 

 

Question / Proposal

Purpose:

The purpose of my project is to find a suitable NPh adsorbent surface comprised of N-C-TiO2 and synthesized by pyrolysis of microcrystalline cellulose (MCC) and dopamine mixture along with TiO2, which is able to adsorb NPh efficiently and economically from the environment.

                     

                             

Question:

Whether adsorbent surface comprised of N-C-TiO2 and synthesized by pyrolysis of microcrystalline cellulose (MCC) and dopamine mixture along with TiO2 is able to adsorb Nitrophenols (NPhs) efficiently and economically from the environment and which pH, initial concentration of Nitrophenol, adsorbent dose and Nitrophenol kind is this adsorbent perform optimal?

Hypothesis:

I thought  adsorbent N-C-TiO2 will be inexpensive, environmentally friendly, highly porous, and very efficient. Cellulose is an abundant and natural polymer which can be modified to synthesize nitrogen-doped carbonaceous material incorporating titanium dioxide (N-C-TiO2).The concept of using N-C-TiO2 material is attractive and novel as nitrogen-doped material interacts with NPhs via hydrogen bonding and TiO2 increases the surface area of the carbonaceous material.  Dopamine is a biomolecule that undergoes self-polymerization in air to form polydopamine, a bioinspired polymer and contains a nitrogen atom that acts as nitrogen source for synthesizing nitrogen-doped carbonaceous materials. Once synthesized, the materials were characterized using different analytical techniques and employed to remove different NPhs.

                              

                                          Figure 4: Schematic diagram of adsorbent N-C-TiO2

Null Hypothesis (H0): There will be no significant adsorption improvement with N-C-TiO2 compared other available adsorbent

 Alternative Hypothesis (HA): N-C-TiO2 will show significantly higher adsorption capacity compared other available adsorbents.

Research

Water is a basic human need but unfortunately 1 in 9 i.e. 844 million people worldwide don’t have access to safe drinking water (Drinking-water, n.d.; Facts and statistics | WaterAid Global, n.d.). Even 63 million Americans are exposed to contaminated water (Philip et al., 2017). The daily needs of people cause the increase in the demands of certain chemicals such as pesticides, herbicides, insecticides, synthetic dyes, pharmaceuticals, and personal care products etc. which have increased the exposure of organic pollutants in water resources (Dieckmann and Gray 1996).The U.S. Environmental Protection Agency (EPA) reported 4.9 billion pounds of pesticide usage/year in the USA alone (Gilbert, 2014). These chemicals undergo hydrolysis and biodegradation to produce NPhs (Mehrizad et al. 2012), which get added into water sources. An Environmental Protection Agency survey revealed, 14.6% wells in USA contain pesticide at levels higher than federal minimum reporting limit (EPA NPS, n.d.).

                          

                       

  According to the Environmental Protection Agency (EPA), 4-NPh is one of the most toxic pollutants released to the environment (Environmental Protection Agency; 1980). Small dose of NPh can cause eye, nose, and throat irritation and nausea where as high dose of NPh can cause kidney and liver damage and Inhibits the DNA repair synthesis.  Therefore, the removal of NPhs from waste water sources is very important.                             

 Currently, many scientists are working on different technologies for 4-NPh remediation. For example, 4-NPh can be removed by activated tea waste adsobtion (Ahmaruzzaman and Laxmi Gayatri 2010), solvent extraction (Shen et al. 2006), biodegradation (Tomei et al. 2009), photocatalytic degradation (Bhar et al. 2015), and oxidation with ozone (Mehrizad and Gharbani 2014) etc.

Among these techniques, the adsorption technique is so far the best as it is economical (Mehrizad and Gharbani 2016). For the adsorption of 4-NPh from water, different adsorbents such as activated carbon (Ayral et al. 2010),  metal organic frameworks (MOFs) (Liu et al. 2014), activated tea waste (Ahmaruzzaman and Gayatri 2010), etc. have been used. Activated carbon is porous and has high adsorption capacity but is expensive and the adsorption process is irreversible (Ayral et al. 2010). The use of organic ligands to synthesize MOFs can be costly, to some extent toxic, and unstable in aqueous environment (Zhou et al. 2012).

Therefore, the type of adsorbent should be inexpensive, highly porous, environmentally friendly, readily available, and reversible. Recently, numerous approaches have been studied in developing adsorbent from environmentally benign materials. However, to the best of my knowledge, the use of N-doped carbonaceous material using cellulose and metal oxide nanoparticles as an adsorbent of NPhs has not been explored. Cellulose is an abundant and natural polymer which can be modified to synthesize nitrogen-doped carbonaceous material incorporating titanium dioxide (N-C-TiO2). The concept of using N-C-TiO2 material as adsorbent is attractive and novel as nitrogen-doped material interacts with NPhs via hydrogen bonding and TiO2 increases the surface area of the carbonaceous material.  Dopamine self-polymerized in air to form polydopamine, a bioinspired polymer and contains a nitrogen atom that acts as nitrogen source for synthesizing nitrogen-doped carbonaceous materials.

                                

 

Method / Testing and Redesign

Methods:

Synthesis of N-C-TiO2 adsorbent:

Required amount of microcrystalline cellulose, dopamine hydrochloride, and TiO2 were mixed in 25 mL of DI water and stirred for 3 h in air under ambient conditions. The pH of the mixture was maintained at 8.0 throughout the reaction. The resulting mixture was filtered and washed with DI water to remove any impurities. The dried solid sample was pyrolyzed at 500 °C inside using a tube furnace for 2 h. Similarly, nitrogen doped carbonaceous (N-C) adsorbent with precursor MCC and dopamine hydrochloride was synthesized. Synthesis is shown in Figure 5.

                        

General:

Surface characterization of N-C-TiO2 adsorbent was carried out by using a JEOL scanning electron microscopy (SEM) instrument with an EDAX Pegasus energy dispersive spectroscopy (EDS) system and transmission electron microscopy (TEM). Surface area and porosity measurements of the adsorbent were performed using Brunauer Emmett Teller (BET) instrument. Ultraviolet-visible (UV-Vis) absorption spectra of NPhs were recorded using a Varian Cary 5000 UV-Vis-NIR spectrometer at their corresponding absorption maxima (λmax).

Batch adsorption:

The effect of different parameters such as pH, contact time, concentration of NPh, and doses of adsorbent on NPh adsorption was studied in the batch experiment. At first, a 50 mL 4-NPh solution (10-4 M) in pH 3.0 phosphate buffer, 10 mg of N-C-TiO2 was suspended. Then the mixture was stirred continuously for total 240 min under dark condition experiment was performed in dark with continuous stirring for 240 min. During this period, about 3 mL of sample was taken out in every 30 min. After filtration, samples were analyzed by UV-Vis spectrometer. The percentage removal of NPh was calculated by using Eq. (1).           

    

At is the absorbance at time t that corresponds to NPh concentration at time t and absorbance, Ao, corresponds to the initial NPh concentration using:

Adsorption isotherms:

    Adsorption isotherms describe the way of absorbing the molecules on the adsorbent surface. Isotherms studies were carried out with different initial NPh concentrations ranging from 4×10-5 to 10×10-5 M. The linear forms of the Langmuir adsorption isotherm equations were fitted to the experimental data.

The Langmuir isotherm is based on the monolayer and uniform adsorption site. The Langmuir adsorption equation is given below (Eq. 2).

                                         

Where, KL (L·mg-1) and qm (mg·g-1) are the Langmuir equilibrium constants calculated from the intercept and slope of the straight line, respectively, from the plot of Ce/qe vs Ce. qm is related to the theoretical monolayer capacity and KL is related to the affinity of binding sites and energy of adsorption.

Reusability:

 The reusability of the adsorbent was also tested by washing the adsorbent after each adsorption cycle with 25 mL of methanol. Methanol solution was added to a beaker containing adsorbent after one each cycle to remove the adsorbed 4-NPh. The mixture was sonicated for five minutes and kept for one-night to remove any residue of 4-NPh completely from the adsorbent. Methanol was centrifuged to collect the adsorbent. Finally, the adsorbent was dried on using a hot plate at 50 °C for 15 minutes for further testing.

Results

The SEM images of N-C-TiO2 were presented in the Figure 6, which revealed the formation of highly porous graphitic carbon with some agglomeration. The red circles in Figure 6a, indicated the deposition of TiO2 on carbons surface.  From TEM images (Figure 6b), it was observed that spherical shaped TiO2 nanoparticles were deposited uniformly on the carbon surface

                  

From BET study, surface area, average pore width and diameter were determined to be 208.78 m2·g-1, 34.49 and 114.08 Å respectively. Therefore, the pore size of the adsorbent was large enough for trapping NPh molecules and enhancing the adsorption capacity.

The change of UV/Vis spectra of 4-NPh adsorption over the time is shown in Figure 7.

       

              

Percent removal of 4-NPh has been calculated next by varying the pH, initial concentration of NPhs, and adsorbent dose.

Figures 9 shows the effect of pH on the removal of 4-NPh from the impure water. The percentage removal of 4-NPh maximized at pH 3.0. At pH 3.0, the highest percentage of removal (80%) was observed for 50 mL of 10-4 M 4-NPh solution after 240 min using 10 mg of the adsorbent (N-C-TiO2).

             

To study the effect of the different doses of the adsorbent, 4-NPh at pH 3.0 was treated with varying amount of N-C-TiO2 adsorbent ranging from 10 to 30 mg (Figure 10). It was observed that rate of adsorption increased proportionally with the amount of adsorbent used as the increased number of adsorption sites and other active atoms such as N atoms with increasing the dose.

                  

The effect of concentration of 4-NPh on the rate of adsorption: It was found that the rate of adsorption increased with decreased concentration of 4-NPh as shown in Figure 11. This increased percentage of removal in low 4-NPh concentration was due to less competition for adsorption among the 4-NPh molecules as the availability of the number of pores and the active elements per 4-NPh molecule were increased.

           

Adsorption of different NPhs such as 4-NPh, 3-NPh and 2,4-DNPh using N-C-TiO2 was studied at pH 7.0 (Figure 12). In the case of 4-NPh, 48% was removed after 4 h. On the other hand, only 40% and 21% of 3-NPh and 2,4-DNPh were removed after 4 h, respectively. 

                  

 The fitting of isotherm data was tested using Langmuir isotherm models. The change of UV/Vis spectra of 4-NPh adsorption over the time is given shown in Figure 13 a. while corresponding Langmuir isotherm plot is shown in Figure 13 b.

                  

 

Langmuir isotherm model with R2 value (0.998) showed a good fitting; indicating the formation of a monolayer of 4-NPh on the adsorbent surface (Li et al. 2009). This also confirmed that the adsorption was due to both physical adsorption and chemisorption (Bubba et al. 2003). Langmuir adsorption capacity (qm) was calculated to be 52.91 mg·g-1.

It is quite significant compared to other adsorbents as shown in Table1.  

                

Multi-cycle use of the adsorbent is beneficial and economically attractive. Figure 14 showed that even after the fifth cycle; the adsorbent was still efficient at removing 4-NPh. 

               

Conclusion

Conclusion

Doped carbonaceous material was synthesized and tested its efficacy for the adsorption of NPhs. Initial pH of the solution was found to be primary factor for 4-NPh removal. From the data studied, it was clear that an acidic pH was suitable for greater removal of 4-NPh. At pH 3.0, on average, 80% of 10-4 M 4-NPh was adsorbed using 10 mg of N-C-TiO2 adsorbent. The presence of nitrogen atom enhanced for the adsorption of molecular 4-NPh by providing hydrogen bonding through hydrogen bond formation. The use of TiO2 was found to further enhance the adsorption capability. From the reusability data, it was found that the adsorbent can be used for multiple cycles for effective removal of 4-NPh. Furthermore, it was found that 4-NPh was easy to remove compared to other NPhs. To the best of my knowledge, this material was successfully tested for the first-time use in the abatement of aqueous NPhs by adsorption. Adsorption of 4-NPh showed the better fitting of Langmuir isotherm plot with R2 value of 0.9981 and absorption capacity of 52.91 mg·g-1which is quite significant compared to other adsorbents currently available.

Implications

This experiment showed that the adsorbent is efficient, cost effective, environment friendly and sustainable: Adsorption capacity of this adsorbent is “52.91 mg·g-1”, whereas “30 ng NPhs/liter of water” is considered high dose, which can cause great harm to humans. Therefore, one gram of this adsorbent can clean millions of liters of water. My adsorbent is reusable up to 5 cycles. This adsorbent is made of abundant inexpensive environment benign materials (Cellulose: TiO2: Dopamine = 5:5:1).  At $6.75 per kilogram raw material cost, it proved to be a real good and economically viable solution comparing to the current alternatives.

New Questions:

In the future, I like to use this material for the removal of different organic pollutants.  The N-C-TiO2 adsorbent was synthesized as a novel material using environmentally benign and green precursors such as cellulose and dopamine. I will explore similar kind of adsorbent with different metals. I also like to study the kinetics of adsorption of 4-NPh on the adsorbent as well as use this adsorbent as photocatalytic materials for the complete removal of the pollutant in future.

About me

We are a small family of four – My parents, my little brother and me. I am from a small city of Little Rock, Arkansas.

 

I love volunteering for different social causes. While in 8th grade, one of my close friends fell victim of mental depression. I started a campaign against mental. I volunteered numerous hours in local hospitals with dying patients, in food banks packing food and many more.

 

When I was seven, My dad and I were fixing up the Christmas train. It had two moving parts which were not connecting properly. My dad gave up but I managed to make it work. My engineer daddy quipped that I will make a better engineer than him. From that moment I started loving science and engineering though didn’t have much clue what they were.

 

I admire Albert Einstein the most. It always astonished me how he thought of the concept of relativity at the early 20th century. Stephen Hawking is another of my favorite, not only for his great contributions to science but how he overcame all physical obstacles.

 

After completing my graduation in computer science, I would like to pursue further studies and research and become a professor in the future.

 

I already have been accepted in several out of Arkansas colleges but that also means out-of-state tuition fees. This scholarship money will go a long way to pay for some of that and reduce my student loan amount. It will help me fulfill my dream.

Health & Safety

Risk and Safety:

All material safety data sheets (MSDS) were consulted prior to using any reagents. A graduate student supervised all reactions. All reactions were performed in well ventilated fume hoods and protective equipment (safety googles, gloves and laboratory coats) were used at all times during reactions or measurement.

Research Laboratory Location:

University of Arkansas in Little Rock
Describe the Research Location Labs FH 519 and CINS 321
University of Arkansas in Little Rock
2801 S. University Ave., Little Rock, AR 72204

Mentor:

Dr. Anindya Ghosh
Professor
University of Arkansas at Little Rock
axghosh@ualr.edu
 

I was directly supervised by Dr. Anindya Ghosh and an experienced graduate student.

Supervisor:

Ambar Rangu Magar
Graduate Student
University of Arkansas at Little Rock

Risk Assessment Form

Lab Rules and Regulations Agreement

Bibliography, references, and acknowledgements

Research Location:

University of Arkansas in Little Rock

Labs FH 519 and CINS 321
University of Arkansas in Little Rock
2801 S. University Ave., Little Rock, AR 72204

Mentor:

Dr. Anindya Ghosh                                                                                                                                                Professor
University of Arkansas at Little Rock

Acknowledgements:

I am really grateful to Dr. Anindya Ghosh for giving me the opportunity to do my research project at UALR lab and guiding me in every way possible. I would also like to thank Ambar Rangu Magar for helping me throughout the execution of the project. I also like to thank my school science teachers, Dr. Marris and Mr. Foley.

 

  • Developing / Initiating the purpose:I wanted to develop an inexpensive and environmentally friendly way to remove pollutants from water. I was then guided by Dr. Ghosh, my mentor, in searching the literature for published work in the similar field of science. Based on that I developed the goal of my project: To find a suitable adsorbent, which would be able to efficiently remove Nitrophenols from water and will be inexpensive.
  • Designing the procedures: I was guided in searching the literature for similarly published work. I planned to synthesize nitrogen atom doped renewable polymer cellulose. I suggested/designed how I should plan on testing the materials to check the removal efficiency.
  • Implementing the procedure: Due to safety concerns, synthesis and testing of the catalysts were done under the supervision of a graduate student, Ambar Rangu Magar. However, I was completely involved in all steps such as synthesis, characterizations and testing.
  • Gathering / Recording data: I actively participated in the NPhs absorption study. I was assisted in using the UV Spectrometer and various other equipment by graduate student Ambar Rangu Magar, while Dr. Ghosh served as the general facilitator of my project at UALR. 
  • Analyzing data: I was able to analyze and interpret the data with some guidance. I obtained some exciting data for the project and found that I can remove nitrophenol easily using my adsorbent N-C-TiO2. The material can be regenerated and reused for the Nitrophenol removal purposes. 
  • Formulating conclusions: I formulated conclusions after analyzing data

 

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