Evaluation of Cold Dew Wind Monitoring Precision for Late Rice Based on CLDAS Data in Guangxi

Yanli CHEN Yan HE Jianfei MO Yongming LUO Meihua DING

AbstractWater pollution caused by ammonia nitrogen is of major concern in many parts of the world due to the danger it poses to the environment and human health. This study focuses on the development of an inexpensive and environmental adsorbent by means of modified corncob. The objective of this paper was to investigate the adsorption behavior of NH+4N from wastewater by modified corncob. Corncob was modified with KMnO4. The physicochemical properties of modified corncob were characterized by fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). It was found that the adsorption capacity of corncob was improved significantly after modification with KMnO4. The pH significantly affected the adsorption efficiency of modified corncob to NH+4N. The best pH value of corncob adsorbing NH+4N was 7. The coexistence of Na+ had a significant effect on the adsorption of NH+4N. The adsorption process of modified corncob to NH+4N followed the pseudofirst order kinetic model. Langmuir model could well simulate the adsorption behavior of NH+4N on modified corncob. The maximum adsorption capacity of NH+4N on modified corncob can reach 4.85 mg/g. The adsorption process of NH+4N was monolayer adsorption. Moreover, modified corncob adsorbed NH+4N was fertilizer conservation especially for nitrogen. The utilization of modified corncob with NH+4N adsorption in the farmland could promote the gradual release of nutrients, thus providing nutrients for plant growth. It was proposed that if combined with biological method, the amount of removed NH+4N from wastewater could be increased significantly.

Key wordsModified corncob; Adsorption; NH+4N; Wastewater



Received: June 25, 2018Accepted: September 29, 2018

Ruigang WANG (1978-), male, P. R. China, associate professor, devoted to research about waste treatment and recycling.

*Corresponding author. Email: 122822404@qq.com.

Ammonia nitrogen (NH+4N) pollution has become a serious environmental problem in recent years. NH+4N is formed by the decomposition of urea, proteins, and other nitrogencontaining substances［1］. It has been found to produce various types of wastewater from agricultural, industrial and domestic activities. NH+4N is found in steel, glass, refining, inorganic chemicals, chemical fertilizers, ferroalloy, meat processing and feed production and other industries. The presence of a great deal of NH+4N in the lakes, rivers and bays which flow slowly can easily bring about water eutrophication, which result in foul smell and dissolved oxygen content decreased for aquatic species. At the same time, a great deal of fish might die and even lakes might perish, thereby hindering the sterilization of water supplies［2-3］. Therefore, it is significant to study the effective removal NH+4N method from wastewater. In recent years, a great deal of research has been carried out by diverse experts.

Although several methods have been used to remove NH+4N from wastewater, including biological nitration, selective ion exchange, stripping, adsorption, chemical precipitation and breakpoint chlorination［4-5］, among which, adsorption is considered to be a perspective technique. Several types of NH+4N adsorbent have been measured by far, but most of which remain at laboratory scale applications［6］. In recent years, more and more attention has been paid to biological adsorbent for adsorb pollutants from wastewater, and agricultural and sideline products have been used as biological adsorbents. Some common agricultural and sideline products which can be used as biological adsorbent are corncob, rice husk and so on［7-8］.

The annual output of corncob in China is huge and has great application prospects. In this study, KMnO4 was used to modify corncob, and its adsorption characteristics of NH+4N from wastewater were investigated. KMnO4 has strong oxidation, on the one hand, KMnO4 can be a redox reaction with the reduction of organic functional groups of corncob, increasing the number of oxygencontaining functional groups on the surface of corncob［9］, thereby enhancing the adsorption capacity of corncob. On the other hand, the corncob surface has a new biological MnO2 (δMnO2) generated, and the new ecological MnO2 can adsorb heavy metal cations in wastewater［10］, its large surface area, can provide a large number of active sites［11］, the adsorption ability of corncob can also be enhanced. A series of batch experiments were conducted to study the adsorption behavior of modified corncob to NH+4N from wastewater. Effect of sodium ion and pH on the adsorption capacity of modified corncob was shown in this article. Moreover, the equilibrium time was measured in order to investigate adsorption kinetics and adsorption isotherms about the adsorption of a NH+4N by modified corncob in this study, which is helpful to understand the adsorption performance and provide the idea for improving the adsorption capacity.

Materials and Methods

Chemicals

In this study, all chemical reagents were purchased from Sinopharm Chemical Reagent Co. Ltd., China only if mentioned. They were all analytical grade and were not purified for further use. The nitrogen gas (more than 99.9 %), stored in the steel cylinder, was purchased from Shanxi Jinlong Taida Gas Co. Ltd., China. 3.819 g of NH4Cl (dried in an oven at 105 ℃ for 2 h) was accurately weighed, then dissolved and diluted to 1 000 ml with distilled water in a volumetric flask, so the stock solution of NH+4N with a concentration of 1.0 g/L was obtained. Adsorption wastewater was prepared by diluting the stock solution of NH+4N to the required concentrations with distilled water. The pH value of the wastewater was measured by digital pH meter (Shanghai Instrument and Electrical Scientific Instrument Co. Ltd., China) and adjusted by adding hydrochloric acid and sodium hydroxide solutions.

Corncob modification

The corncob sample used in this study was collected from Taiyuan area, Shanxi Province, China. Its average moisture and ash contents were 14.72% and 8.96 %, respectively. In order to removal impurities, the corncob was firstly washed for three times with tap water, three times with distilled water and was dried in an oven (Wujiang Huali Heat Treatment Equipment Co. Ltd., China) at 105 ℃ for 2 h to a constant weight. The material used as the front of modified corncob was stored in dry and aerated place before use. 20 g of dried corncob was packed tightly into the stainless steel pyrolysis device (Hefei Kejing Material Technology Co. Ltd., China), under inert nitrogen atmosphere with the flow rate of 2.5 m3/h (in the anaerobic state). The stainless steel pyrolysis device in the muffle furnace (Shanghai Bozhen Instrument Equipment Co. Ltd., China) heated at 400 ℃ for 2 h, then cooled to room temperature, and after the corncob was removed, it was then grinded and sifted through the 0.25 mm aperture sifter.

3 g of corncob and 6 g of KMnO4 was placed in a 500 ml conical flask, and then 250 ml of distilled water was added. The conical flask was placed in a thermostatic water bath oscillator (Jiangsu Jintan Youlian Instrument Co. Ltd., China), and oscillated at 25 ℃ and 150 r/min for 24 h, and then the modified corncob was washed with distilled water and dried in an oven at 65 ℃ for 24 h, and then bagged for use.

Modified corncob characterization

The surface chemical groups of modified corncob were studied in the region of 400-4 000 cm-1 utilizing a fourier transform infrared spectrometer (FTIR IRPrestige21, Japan). Scanning electron microscope images were recorded using a scanning electron microscope (FESEM Hitachi, Japan).

Adsorption studies

The experiments were carried out in 250 ml conical flasks containing a certain amount of modified corncob and 100 ml of wastewater with a known NH+4N concentration, pH value and temperature. The conical flasks were shaken at 150 rpm in a thermostatic water bath oscillator. After a certain contact time, all the wastewater samples were filtered through 0.45 μm poresize filters. The concentration of NH+4N in the filtrate was analyzed using Nesslers reagent spectrophotometry (Shanghai spectrum Instrument Co. Ltd., China) at 420 nm［12］. The amount of NH+4N adsorbed on modified corncob was calculated from the difference between the initial concentration of NH+4N and the residual concentration of NH+4N in the wastewater. The adsorption capacity of modified corncob (qe mg/g)) can be calculated by the following formula: qe=V (C0-Ce)/m, where qe is the amount of NH+4N adsorbed by modified corncob (mg/g), V is the volume of the wastewater (L), m is the weigh of modified corncob added into the wastewater (g), and C0 and Ce (mg/L) represent the initial and equilibrium concentrations of NH+4N, respectively.

Results and Discussion

Characteristics of modified corncob

Energy spectrum analysis of modified corncob was analyzed by X ray energy spectrometer (Thermo Fisher Scientific Co. Ltd., America). The results showed that modified corncob mainly contained C, O, and a small amount of Si, Cl, Ca and Mn elements. The presence of Mn element showed that the manganese oxide on the surface of modified corncob, it should be the new ecological MnO2 (δMnO2) produced by KMnO4 reduction. The new ecological MnO2 can promote the adsorption of NH+4N by corncob［13］. The FTIR spectra of corncob before and after modification are shown in Fig. 1.



Fig. 1FTIR spectra of corncob before and after modification

Fig. 1 shows the OH stretching vibration absorption peak of COOH of corncob cellulose located around 3 416 cm-1, and the stretching vibration absorption peak of C=C and C=O of corncob cellulose located around 1 495-1 645 cm-1, and the symmetrical flexural vibration absorption peak of CH3 of corncob cellulose located around 1 372 cm-1, and the vibration absorption peak of CO of lignin hydroxyl radical of corncob lignin located around 1 231 cm-1.

After modification, the peaks of corncob were slightly displaced. In addition, the absorption peak located around 3 416 cm-1 become wide and strong, indicating that the number of OH groups of corncob increased. The absorption peak located around 1 231 cm-1 decreased, indicating that modified corncob had a certain degree of degradation (breakage). There was a strong absorption peak located around 508 cm-1, which was the stretching vibration absorption peak of the MnO bond in the new ecological MnO2 (δMnO2)［14］, This further confirmed the formation of new ecological MnO2. The increase of the number of OH groups and the formation of new ecological MnO2 enhanced the adsorption capacity of corncob.

The surface morphology of modified corncob was observed by scanning electron microscopy at 20 000 and 50 000 times magnification, as shown in Fig. 2.



Fig. 2SEM images of modified corncob

Fig. 2 shows the corncob surface distribution of many cavities, there were many small particles at the same time, and these small particles may be a new ecological MnO2 (δMnO2). The existence of particles and cavities led to modified corncob having larger surface area, and provided a vast number of active sites for adsorption.

Comparison of adsorption effects before and after modification

In order to compare adsorption effects before and after modification, 0.5 g of corncob before and after modification were respectively added into the wastewater of which the volume, initial concentration, temperature and pH value were 100 ml, 40 mg/L, 25 ℃ and 6, respectively. And then they were put into a thermostatic water bath oscillator to be oscillated under constant temperature and 150 r/min until adsorption equilibrium.

The results showed that the adsorption capacity of NH+4N on corncob increased from 0.68 to 3.89 mg/g after KMnO4 modification, which was 5.72 times of the amount of adsorption before modification. Therefore, corncob modified by KMnO4 was an effective modification method.

Effect of influence factors on adsorption

Effect of pH

In order to examine the removal of NH+4N within a certain range of initial pH, the initial pH value of wastewater was adjusted by hydrochloric acid and sodium hydroxide. No substance was added to adjust the pH value in the following adsorption process. Other basic conditions including wastewater volume, temperature, initial concentration of NH+4N and the dosage of modified corncob were 100 ml, 25 ℃, 40 mg/L and 0.5 g, respectively. And then they were put into a thermostatic water bath oscillator to be oscillated under constant temperature and 150 r/min until adsorption equilibrium. The results were shown in Fig. 3.

Fig. 3 shows that with the initial pH value increased from 2 to 7, the adsorption capacity of NH+4N on modified corncob increased. When the initial pH value was 7, the adsorption amount reached the maximum value 4.43 mg/g. When the initial pH value increased from 7 to 9, the adsorption capacity of NH+4N on modified corncob decreased significantly. This was mainly due to the competitive adsorption between H+ and NH+4 under acidic conditions. In acidic solution, a smaller pH was more unfavorable to the adsorption of NH+4 by modified corncob. In addition, there was the following reaction in wastewater: NH+4+OH-=NH3?H2O=NH3+H2O. In acid or neutral wastewater, ammonia nitrogen mainly existed in the form of NH+4N, which was beneficial to the adsorption of ammonia nitrogen. In alkaline wastewater, ammonia nitrogen mainly existed in the presence of NH3•H2O, which was not conducive to the adsorption of ammonia nitrogen.



Fig. 3Effect of pH on adsorption of NH+4N

Considering the experimental results, and as we all know that most microorganisms show strong activity in a neutral environment, we usually use the combined technology with biological method to treat NH+4N, and therefore, 7 was selected as the initial wastewater pH value of this study.

Effect of Na+

During the examination effect of Na+, different dosages of sodium chloride were added into the wastewater of which the initial concentration of NH+4N, wastewater volume, pH value, temperature and the dosage of corncob were 40 mg/L, 100 ml, 7, 25 ℃and 0.5 g, respectively. And then they were put into a thermostatic water bath oscillator to be oscillated under constant temperature and 150 r/min until adsorption equilibrium. The results are shown in Fig. 4.



Fig. 4Effect of Na+ on adsorption of NH+4N

Ruigang WANG et al. Studies on Removal of Ammonia Nitrogen From Wastewater Using Modified Corncob

Fig. 4 shows that with the increase of Na+ concentration in wastewater, the adsorption capacity of NH+4N on modified corncob decreased sharply. When the concentration of Na+ was more than 0.05 mol/L in the wastewater, the modified corncob will no longer adsorb NH+4N. The experimental results showed that there existed competitive adsorption between Na+ and NH+4N in wastewater.

Adsorption kinetics

It was important to determine the equilibrium time of adsorption when researching on adsorption kinetics and isotherms. In this study different adsorption time was designed ranging from 0.5 h to 50 h. In order to determine the equilibrium time, 0.5 g of modified corncob was added into the wastewater, of which the concentration of NH+4N, wastewater volume, initial pH value and temperature were 40 mg/L, 100 ml, 6 and 25 ℃, respectively. Then it was put into a thermostatic water bath oscillator to be oscillated under constant temperature and 150 r/min. The results were shown in Fig. 5.



Fig. 5Adsorption kinetic curve of NH+4N

It could be observed from Fig. 5 that the adsorption of NH+4N on modified corncob followed the law of rapid adsorption and slow balance. At the initial stage of adsorption, the adsorption rate was large. With the reaction, the adsorption rate decreased gradually, and the adsorption capacity tended to balance. This was because there were a large number of adsorption sites on the modified corncob surface in the early stage of adsorption. With the prolongation of reaction time, the adsorption sites were gradually occupied, and the adsorption was close to saturation. The adsorption of NH+4N to the modified corncob reached equilibrium at 12 h, and the equilibrium adsorption capacity was 3.89 mg/g. Therefore, 12 h was taken as the equilibration time in this study. Moreover, the experimental data was fitted with the pseudofirst order kinetic model and the pseudosecond order kinetic model［15］. The kinetic constants obtained from this study are shown in Table 1.

Table 1Fitting constants of adsorption kinetic model

ItemTemperature∥℃

Pseudofirst order kinetic model

qe∥mg/g K1∥min-1R2

Pseudosecond order kinetic model

qe∥mg/g K2∥mg/(g?min)R2Experimental value

NH+4N303.870.742 30.9864.160.261 80.9683.89

It could be obtained from Table 1 that the correlation coefficient R2 of the pseudofirst order kinetic model was 0.986, far greater than the correlation coefficient of pseudosecond order kinetic model. In addition, according to pseudofirst order kinetic model, simulation adsorption of NH+4N(qe) was 3.87 mg/g, which was very close to the experimental value 3.89 mg/g. It was showed that the firstorder kinetic model can better describe the adsorption of NH+4N by modified corncob.

Adsorption isotherm

In order to study the adsorption isotherms, a batch of conical flasks were prepared with 100 ml wastewater of which the initial concentrations of NH+4N were different ranging from 5 to 40 mg/L. The pH value of the solution was adjusted to 6 by hydrochloric acid and sodium hydroxide before adding 0.5 g of modified corncob into the solution. Then they were put into a thermostatic water bath oscillator to be oscillated under 25 ℃ and 150 rpm. The adsorption isotherms of modified corncob to NH+4N are shown in Fig. 6.

It could be observed from Fig. 6 that the adsorption capacity of NH+4N on modified corncob increased with the initial concentration of NH+4N in the solution, but its range of increase was smaller and smaller. The experimental data were fitted by Langmuir model and Freundlich model, and the results are shown in Table 2.



Fig. 6Adsorption isotherms of NH+4N on modified corncob

Table 2Fitting constants of Langmuir model and Freundlich model

ItemTemperature∥℃

Langmuir model

qmaxKLR2

Freundlich model

1/nKFR2

NH+4N304.850.422 60.9630.2122.257 00.917

It can be seen from table 2 that the Langmuir model can better simulate the adsorption process of NH+4N on modified corncob, which showed that the adsorption process of NH+4N was monolayer adsorption, and there was no interaction between ions. In the Freundlich model, when 0.1<1/n< 1, it indicates that the adsorption is easy to carry out. In the experiment, the fitting constants 1/n of Freundlich model was between 0.1 and 1, which showed that modified corncob was easy to adsorb NH+4N. From the Langmuir model, the maximum adsorption capacity of ammonia nitrogen on modified corncob can reach 4.85 mg/g.

According to relevant literature reports, the maximum adsorption capacity of NH+4N on bamboo biochar can reach 0.85 mg/g［16］. It can be seen that the modified corncob had better adsorption effect.

Conclusions

After modification with KMnO4, the adsorption capacity of corncob was improved significantly. When the temperature, initial pH value, initial concentration of NH+4N were 25 ℃, 6, 40 mg/L, respectively, the adsorption capacity of modified corncob to NH+4N was 5.72 times of that before modification. After modification, the number of OH groups in corncob increased and new MnO2 was generated on the surface of the corncob, which provided more active sites for NH+4N, thus enhancing its adsorption capacity. pH significantly affected the adsorption efficiency of NH+4N on modified corncob. The coexistence of Na+ had a significant effect on the adsorption of NH+4N. The adsorption process of NH+4N on modified corncob followed the pseudofirst order kinetic model. Langmuir model could better describe the adsorption behavior of NH+4N on modified corncob. From the Langmuir model, the maximum adsorption capacity of ammonia nitrogen on modified corncob can reach 4.85 mg/g. The adsorption process of NH+4N was monolayer adsorption. Although there was little complete removal of NH+4N by adsorption, modified corncob adsorbed NH+4N was a fertilizer conservator, especially for nitrogen. The extensive utilization of modified corncob with NH+4N adsorption in the farmland could promote the gradual release of nutrients, so as to provide nutrients for plant growth. Above all, if combined with biological method, the amount of removed NH+4N from wastewater could be increased significantly.

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