Facile Synthesis of Silver Nanoparticles from Mesquite Tree Leaves for Refinery Wastewater Treatment

. Green synthesis of nanoparticles has gained significant importance, and it has become the one of the most preferred synthesis methods. Mesquite or Prosopis juliflora is an aggressive, thorny tree is threatening the ecology and has a negative impact on the environment, and animal health as it absorbs nutrients from soil, prevents the water flow in aflaj . Therefore, the purpose of this research is to synthesize silver nanoparticles (AgNPs) from mesquite leaf extract by green extraction technique for the refinery wastewater treatment applications. The AgNP’s are characterized by Fourier Transform Infrared spectroscopy (FTIR), Energy Dispersive X - Ray Analysis (EDX), UV-visible spectroscopy, and Scanning Electron Microscopy (SEM) analyses to determine the functional groups, morphology, and elemental composition. The synthesized AgNP’s are utilized for the treatment of refinery wastewater by performing a series of experiments by varying the processing parameters (pH, mixing duration, agitation speed, and dosage of nanoparticles) and the optimum treatment conditions were established. This research aims to convert the mesquite tree leaves into a value added product, AgNP’s for the refinery wastewater treatment, thereby reduce its impact on the environment. Additionally, this research is in line with the United Nations Sustainable Development Goals (UNSD-6, clean water and sanitation) and also supports the Oman Vision 2040.


Introduction
The population growth, swift escalation in economic growth, and improved living standards has increased the consumption of fresh water [1].The availability of clean water is a global issue, and conventional techniques have many shortcomings in terms of cost, requirement of further purification and generation of solid waste and their disposal issues.Water scarcity is one of the worldwide problems and it is expected to worsen in the ensuing years, due to natural as well as manmade factors such as droughts, industrial revolution, population growth, and rapid urbanization.Prosopis juliflora, sometimes known as the mesquite tree, is a common tree in the Arab world that has a number of detrimental consequences on human culture and other living things.The mesquite trees kill native plants, and their leaves contain poisonous chemicals that are harmful to animals, so feeding mesquite fruits to animals causes tooth loss, and eventually lead to death.Also, pollen can induce allergic reactions in some people [2].It is required to have a countrywide drive to eliminate this tree from the different regions of Sultanate of Oman including Ibri, Bahla, and Almasnaah, is being carried out by the Ministry of Environment and Climate Affairs (MECA) and the Ministry of Agriculture and Fisheries [3].The nanotechnology oriented treatment processes are in high demand nowadays due to their specific structural, optical, electrical, and biological features of metal based nanoparticles.Various type of nanoparticles are currently being employed for the industrial scale application are zinc oxide (ZnO), magnetite (Fe3O4), titanium oxide (TiO2), gold (Au), and copper oxide (CuO) [4].Ag-NPs are extensively applied as antibacterial and antimicrobial activity against AgNP's [5].The surface morphological characteristics, film stability, particle size, and zeta potential of metal nanoparticles play a key role in the facile fabrication of AgNP's from plant extract as a stabilizing and reducing agent [6].The physicochemical parameters of mixing duration, initial concentration of silver nitrate etc. are varied to get desired size range of nanoparticles [7,8].
The oil and gas industry requires huge quantities of clean water during the upstream and downstream processing stages and the polluted water discharged from those sections contains aliphatic hydrocarbons, BTEX and PHAs, phenol, and dissolved inorganic compounds [9].The release of toxic pollutants into water bodies leads to the depletion of dissolved oxygen levels and thereby interrupts the survival of microorganisms [10,11].The common chemical and physical treatment steps are screening, flocculation and coagulation, sedimentation, filtration, activated sludge processes, fenton process, chlorination, and disinfection etc [12].Effluent water discharged from different processing units contains toxic chemicals and other identifiable contaminants, which lead to health issues on living beings and the surrounding environment.Nanoparticle mediated treatment has gained significant importance, and it has become one of the most preferred techniques in recent times.
AgNP's are successfully applied in the removal of heavy metals, degradation of pesticides, photo catalysis, antibacterial activity studies etc [13].In this research, the treatment of oily wastewater was carried out using silver nanoparticles synthesized from mesquite tree leaves by green extraction technique.This novel treatment will aid in enhancing the pollutant removal efficiency of refinery wastewater in a most effective and environmentally way for sustainable water management.

Materials and Methods
AgNP's are synthesized from mesquite tree leaves by green synthesis technique.The mesquite tree leaves were collected from different regions in the Sultanate of Oman, and thoroughly washed with water to remove the dirt and impurities.Then the resulting leaves were dried under three drying conditions viz.room temperature, sun temperature, and oven temperature.After the drying process, the leaves were ground into ultra-fine powder for the preparation of extract for the synthesis of nanoparticles.The powdered leaves were mixed with methanol and stirred for 24 h to extract the soluble compounds.The refinery wastewater samples are collected from a leading refinery, Petroleum Development Oman (PDO).
The synthesis process was carried out by reacting mesquite leaf extract and silver nitrate salt solution (1 mM concentration) in the ratio of 1:5 for 24 hours.After 24 hours, the color of the reaction mixture was changed from green to black, which indicates the successful formation of the AgNP's.The resulting mixture was separated using a centrifuge operated at 6000 rpm speed for 10 minutes.The centrifuged product was washed several times with ethanol to remove any loosely bound unreacted materials on the surface of particles followed by drying.The dried powder was used for the SEM, FTIR, and EDX analysis.

Batch treatment of refinery wastewater
The synthesized AgNP's were employed in the batch treatment of refinery wastewater at room temperature (25 ± 2 ºC) by mixing a known amount of AgNP's with 1000 mL of wastewater under specified mixing duration and agitation speed.The treatment of refinery wastewater was performed by varying the solution pH from 2.0 to 10.0, stirring speed from 25 rpm to 150 rpm, mixing period was altered from 20 minutes to 100 minutes and the quantity of the nanoparticles were varied from 0.1 g to 2.0 g.The percentage reductions in various parameters are studied and the best processing time for the removal of pollutant were established.The optimum processing conditions were achieved by calculating the TSS, COD, TDS, DO and turbidity values.The characteristics of refinery wastewater collected from PDO are shown in Table 1.The diagrammatic representation of the entire process of AgNP's synthesis to refinery wastewater treatment is shown in Figure 1.The Figure 2(a), Figure 2(b), Figure 2(c) represents the fresh leaves, dried leaves and the extract respectively.The synthesized AgNP's after centrifugal separation are shown in Figure 3.

Results and discussions
AgNP's were successfully synthesized from mesquite tree leaves by green synthesis.Figure 4 shows the surface structure of silver nanoparticles analyzed using SEM, where it was found that the silver particles are well formed with scattered distribution.Also, the particle diameters are in nanometer scale (less than 100 nm). Figure 5 describes the EDX spectrum of the nanoparticles showing the elemental analysis of the sample contains 61.2 % of Ag.
The bond stretching and functional groups present on the surface AgNP's are determined by FTIR spectroscopic analysis as shown in Figure 6.The representative peaks observed at wave number corresponding to 3512 cm -1 data confirms the occurrence of O-H stretching, which is responsible for reducing the metal ions into AgNP's.The C-H stretching was experienced at a wave number of 2922 cm -1 .The C=O group was observed at 1632 cm -1 and C-C stretching at 1383 cm -1 .These observations are in agreement with the published results [14,15].The treatment of refinery wastewater was performed by altering the effluent pH from 2.0 to 10.0, agitation speed (25 to 150 rpm), mixing duration (20 to 100 minutes) and the quantity of the nanoparticles (0.1 g to 2.0 g).The parameter reductions are monitored at definite time intervals and the best processing conditions for the effective removal of pollutant were established.The outcome of the experimental study is shown in Figure 7, Figure 8, Figure 9 and Figure 10 respectively.From Figure 6, it is observed that the best COD reduction achieved at pH 6.0.The highest reduction in COD was due to the strong electrostatic interaction between the AgNP's and the oppositely charged ions present in the wastewater.At pH 6.0 the particle surface has maximum number of active sites and hence results in excellent adsorption rate due to electrostatic interaction.The adsorption capacity significantly affects the wastewater pH, which may be due to the surface characteristics of the nanoparticles.The pH of the solution has a direct influence on adsorption process and hence pH plays a vital role in the parameter reductions during water treatment.The mixing duration normalizes the coagulation and flocculation processes and the best COD reduction was viewed at 60 minutes of mixing duration.Influence of dosage of AgNP's with pollutant removal are studied at optimized pH (6.0) and mixing duration (60 minutes) by varying the nanoparticle dosage from 0.1 g to 2.0 g.An increased dosage of the nanoparticles may result in extended surface area and therefore lead to improved availability of vacant sites for adsorption process.The variation of agitation speed on pollutant removal was assessed by altering the agitation speed from 25 rpm to 150 rpm keeping the wastewater pH 6.0, mixing duration 60 minutes.The optimal removal efficiency was obtained at a stirring speed of 100 rpm.From the result analysis of the batch studies, significant reductions in COD values are obtained by altering the processing conditions.

Conclusions
In this research, silver nanoparticles were prepared from the leaves of the mesquite tree by green synthesis.The characterization techniques using SEM, EDX, and FTIR analyses confirmed the successful synthesis of AgNP's.The synthesized nanoparticles were employed in the effective treatment of refinery wastewater and the optimum treatment conditions were established.The best processing conditions obtained from the study are pH 6.0, mixing duration 60 minutes, agitation speed 100 rpm and a dosage of 2.0 g.This research validates that AgNP's synthesized from mesquite tree is effective in the treatment of refinery wastewater by adsorption process in an environmentally friendly and most sustainable way.The United Nations sustainable development goals (UNSDG-6 i.e. clean water and sanitation) are directly linked with the research study.This environmentally friendly CSR based project serves our society in accordance with Oman Vision 2040 because it includes provisions for high-quality, pollution-free environments, and a green circular economy that responds to national needs and is consistent with its global orientation.

Table 1 .
Characteristics of refinery wastewater.