Fermented Corn Stalk for Biosorption of Copper ( II ) from Aqueous Solution

Corn stalk is the amplest and inexpensive organic material in Heilongjiang province, China. ,is resource is vicious, causing pollution of the environment. In this present study, an adsorbent is prepared by corn stalk fermentation withAspergillus niger.,e fermentative effects of water content ratio, initial pH medium, temperature, and time were addressed. ,e analysis of factors and orthogonal experiments revealed that the optimum conditions of producing cellulose were solid-liquid ratio of 1 : 5, temperature 28°C, initial pH, and 72 hours. ,e modification mechanism was investigated by using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). ,e biosorption capacity of fermented corn stalk was better than that of raw corn stalk under identical conditions, and this improvement can be ascribed to the enzyme system secretion by A. niger under changing the surface properties of the raw corn stalk. Some of the hydroxyl and carboxyl groups are bounded by cellulose which became free hydroxyl and carboxyl groups with a high ability after adsorption of heavy metals.


Introduction
Heavy metal discharge causes serious environmental problem due to non-biodegradability.But these metals accumulate in living tissues.Copper is widely being used in industrial applications, but its removal and recovery from wastewater is necessary for protection of the environment as well as human health [1][2][3].Several methods, such as chemical precipitation, coagulation, electrochemical treatment, membrane separation, solvent extraction, and ion exchange are available for the treatment of metal-bearing effluents and were applied to reduce the copper concentration of wastewater [4][5][6][7].However, these methods have limitations, including secondary pollution, high-cost and high-energy input with requirement of large quantities of chemical reagents, and poor treatment efficiency at low metal concentration [8].e alternative biosorption methods are exhibited to overcome these drawbacks.
Many studies have focused on the use of agricultural and food industry waste, such as pine cones, shells and powder [9], almond shells [10], tomato waste [11], and Acidosasa edulis shoot shells [12] but biosorbents remove heavy metals from polluted water.Agricultural discard, especially corn stalk (corn is a major crop in Heilongjiang province, providing grain reserves, feed, and industrial raw materials) is an important part of agricultural straw in Heilongjiang province.Another abundant straw waste, rice, was chemically modified and studied as a biosorbent by Vafakhah et al. [13].A mixture of corn stalk and tomato waste was oxidized with nitric acid, used to investigate the adsorption of copper(II) ions.Wang etal.[14] prepared corn stalk-based adsorbents that were modified by Cu (0)mediated reversible-deactivation radical polymerization (Cu (0)-mediated RDRP) and applied to remove metal ions Hg(II).However, chemical treatment (acid or alkali) may create secondary microbiological modification for friendly environment [15].Aspergillus niger can endure and adapt to various environmental conditions.Functional groups on the cell wall of biomaterials such as corn stalk, include hydroxyl, carboxyl, carbonyl, and amine with a high affinity to form metal complexes [16].In this study, we have prepared corn stalk fermentation by using A. niger and have tested the properties of this material as a biosorbent.Although, there are some reports related to chemical modification of corn stalk, to the best of our knowledge, there have been no published work about the biological treatment for this material.
is study determined the optimal culturing conditions for the modification of corn stalk by A. niger and compared biosorption capacity of the fermented corn stalk (FCS) with the untreated corn stalk.Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to probe the properties of the sample.

Materials and Methods
2.1.Microorganism.Aspergillus niger was isolated from nature, the spore suspension was prepared by washing the aerial mycelium with water, and concentration was adjusted up to 10 6 cfu/ml.e spore concentration of the final suspensions was determined by counting in hemocytometer.

Culture Medium.
e corn stalk was washed with distilled water, crushed with a grinder (FW100, Tianjin eis), and sieved with a standard way to obtain powder with a particle diameter of 0.3 mm.en, the powder was dried in an electrically heated blast dry box to a constant weight.

Optimal Culture Conditions
2.3.1.Effect of Solid-Liquid Ratio.In this case, to determine the effect of the solid-liquid ratio, the spore suspension was added into the corn stalk medium at a solid-liquid ratio, ranged from 1 : 1 to 1 : 7 at 28 °C for 72 h.It gives the clear effect of such ratios.

Effect of Initial pH.
In order to figure out the effect of the initial pH, spore suspension was added into the corn stalk medium with the optimal solid-liquid ratio at different pH values of 4.0, 4.4, 5.0, 5.4, 6.0, and 6.4 at 28 °C for 72 h.e pH values were controlled by the addition of citric acid buffer solution (0.2 M).

Effect of Temperature.
Effect of temperature describes the optimal temperature, and spore suspension was added into the corn stalk medium with an optimal solid-liquid ratio and pH under different temperatures (25 °C, 28 °C, 30 °C, 32 °C, and 34 °C) for 72 h.

Effect of Time.
In order to find out the effect of the fermentation time, spore suspension was added into the corn stalk medium with the best optimal solid-liquid ratio, pH, and temperature for different times (48 h, 72 h, 96 h, 128 h, and 168 h), respectively.

Effect of Excess Carbon and Nitrogen Source.
In this case, we have focused on the effects of excess carbon and nitrogen sources after adding into the culture medium.

Biosorption Experiment.
We evaluated the biosorption of the prepared materials, and 0.15 g dry biosorbent material was put into 100 mL of CuSO 4 •5H 2 O solution with an initial Cu 2+ concentration of 20 mg/L.e flasks were shaken at 30 °C with 150 rpm for 30 minutes.e Cu 2+ solution was filtered through a membrane with a pore diameter of 0.45 μm, and analyzed by inductively coupled plasma mass spectrometry to determine the concentrations of the Cu 2+ before and after biosorption.

Regeneration Studies of Fermented Corn Stalk.
To evaluate the reusability, regeneration of the spent adsorbent was studied.At first, 0.1 g of the adsorbent was loaded by 100 mL of 20 mg/L Cu(II) solution.After attaining equilibrium, the exhausted biosorbent was separated from the solution through a membrane with a pore diameter of 0.45 μm.Subsequently, metal ions were eluted by using 0.1 M HCl.
e regeneration of the biosorbent was sequentially operated three times in this way.All the samples were chemically analyzed for metal determination.

Biosorption of a Binary Metal Solution.
ree pairs of competitive biosorption experiments for Cu 2+ and Pb 2+ , Cu 2+ and Ca 2+ , and Cu 2+ and Cd 2+ ions were performed using the biosorbent.e initial concentration of all the heavy metals was 20 mg/L, and the biosorbent dosage was 0.15 g. e metals and biosorbent were shaken together at 30 °C for 30 minutes.

Biosorbents Characterization.
After being dried to a constant weight, the modified corn stalk and raw corn stalk samples were screened by Fourier transform infrared spectroscopy (FTIR 1730 model, PerkinElmer, Inc.,) via KBR tablet method.e scanning electron microscopy (SEM SU8018, Japan) was employed to under seek the morphology of the two kinds of biosorbents.But the dried biosorbent was coated with gold for better contrast.

Effect of Solid-Liquid Ratio.
In solid fermentation culturing, the corn stalk serves as the culture medium because it has almost no water.e requirement for additional water was assayed by changing the solid-liquid ratio.Figure 1 displays the results of testing the effect of different solidliquid ratios of corn stalk on biological modification.e solid-liquid ratio of 1 : 5, yielded the highest biosorption rate.An appropriate amount of water allows nutrients for absorption in the culture medium, but if there is little water, Aspergillus niger cannot absorb nutrients, and growth will be inhibited.If there is too much water, there may be an insufficient concentration of oxygen in the culture medium, and it limits the fermentation.

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E ect of Initial pH.
Figure 2 exhibits the e ect of different pH values of the culture medium on biological modi cation.Citric acid bu er solution was used to control the pH value of the culture medium throughout the solid fermentation process.e pH value varies from 4 to 6.5 against biosorption rate (%).
e corn stalk was better modi ed when the pH value was in the range of 5.0 to 6.0.But the best biosorption was considered at pH 5.4 in our work.It should be noted, without use of the citric acid bu er solution, the pH values change greatly. is phenomenon suggests that production occurs during the modi cation, resulting the decreasing pH value of the culture medium.Hence, the enzyme activity is suppressed by the lower pH and the biosorption rate [17].

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3. E ect of Temperature.Figure 3 displays the e ect of culture temperature on biological modi cation.
e results show that the culture temperature has a great inuence on the solid fermentation process.When the culture temperature is too low, Aspergillus niger hyphae grow slowly, resulting in less cellulose and poor modication.When the culture temperature is too high, Aspergillus niger hyphae grow faster but microbial cell function decreases due to the high temperature, resulting in less cellulose and poor biosorption [18].e best modi cation was observed clearly after controlling the culture temperature at 28-30 °C.

E ect of Time.
Figure 4 presents the e ect of culture time on biological modi cation.
e results showed that the culture time had a considerable in uence on the solid fermentation process.In the early stage of culture, the mycelial growth of Aspergillus niger was slow with cellulose production, resulting in no modi cation of the corn stalk.However, with increased culture time, the nutrients in the culture medium were depleted and other metabolites were increased, decreasing the activity of the cellulose and lowering biosorption [19].It leads to culture time of 72 h, approaching the best modi cation.Advances in Materials Science and Engineering 3 20 mg/L.e asks were shaken at 30 °C with 150 rpm for 30 minutes (under the same condition of biosorption).e Cu 2+ concentration was still 20 mg/L.So, we come to the conclusion that glucose has no in uence in the process of biosorption of Cu(II).Figure 5 depicts the e ect of di erent dosages of (NH 4 ) 2 SO 4 and glucose on biological modi cation.e results show that the increasing supplemental nitrogen has a little e ect on biological modi cation but increasing carbon source causes to decrease the biological modi cation.In addition, the corn stalk is rich in nitrogen and carbon sources.erefore, it does not need to bene t from additional carbon and nitrogen sources.us, our results are clear indication of additional nitrogen and carbon e ects. 1, under the same conditions, the biosorption of dry raw and fermented corn stalk for Cu 2+ was compared.e raw corn stalk has 48.3% biosorption rate and 6.44 (mg/g) of biosorption capacity.But fermented corn stalk possesses 93.5% biosorption rate and 12.47 (mg/g) of biosorption.is result manifests that the biosorption capacity of the fermented corn stalk was improved to nearly twice than that of raw corn stalk.

Regeneration Studies of Fermented Corn Stalk.
e e ect of regeneration cycles of the fermented corn stalk on the biosorption capacity was constantly tested 3 times, and the results are shown in Table 2.It is seen that the biosorption capacities of fermented corn stalk for Cu(II) were almost not a ected, and the regeneration e ciency of the biosorbent was generally high.
e results illuminate that the spent fermented corn stalk could be e ectively regenerated by HCl and reused at least 3 times without decreasing its biosorption capacity signi cantly.

Biosorption of Binary Metal Solutions.
In order to test the selectivity of the fermented corn stalk towards the removal of Cu 2+ , biosorption by introducing other kinds of divalent ions such as Pb 2+ , Ca 2+ , and Cd 2+ was investigated in a single ion solution.When the biosorbent dosage was 0.15 g and the initial concentration was 20 mg/L, the biosorption value of the biosorbents for Cu 2+ was 12.47 mg/g.However, Figure 6 shows when Pb 2+ , Ca 2+ , and Cd 2+ were added, the biosorption capacities for Cu 2+ decreased to 9.98, 8.72, and 9.35 mg/g, respectively.
e biosorption capacities of the biosorbent in the presence of the binary metal mixture were lower than under noncompetitive conditions.

FTIR of Fermented Corn Stalk and Raw Corn Stalk.
Figure 7 presents FTIR spectra of raw corn stalk and fermented corn stalk before and after biosorption.When comparing corn stalk (a) with fermented corn stalk (b), it is clear that there were some similar biosorption bonds.e IR spectrum exhibits some absorption peaks that are characteristic of cellulose at 3430 cm −1 , 2920 cm −1 , 1380 cm −1 , and 1030 cm −1 [20].e IR spectrum of the fermented corn stalk  e IR results show that the enzymes produced by Aspergillus niger degrade some of the celluloses of the corn stalk and oxidize some of the functional groups of the cellulose.Some of the hydroxyl and carboxyl groups bounded by cellulose which become free hydroxyl and carboxyl groups have a high ability to adsorb heavy metals.From curve (c), we see that after biosorption, with the loading of Cu 2+ ions, there was some shift of peaks in 3640, 3448, 2928, 1731, and 1046 cm −1 , so it is presumed that the Cu 2+ metal ion was incorporated with fermented corn stalk biosorbents through interaction with active functional groups such as -OH, C O, -C O, and C C. erefore, one can be concluded that the fermented corn stalk has a variety of functional groups such as hydroxyl and carboxyl, as well as groups which are important sites for metal biosorption [21].

SEM of Fermented Corn Stalk and Raw Corn
Stalk.Figures 8(a)-8(c) shows the SEM micrograph of raw corn and fermented corn stalks.After comparing the surface structure of raw corn with fermented corn stalk, the surface structure of fermented corn stalk was destroyed and the surface compactness was also decreased.
ere are many porosities on the surface of the fermented corn stalk, allowing part of the inner cellulose of the fermented corn stalk to contact enzymes which are produced by Aspergillus niger. is causes the release of more functional groups that improve the ability to adsorb heavy metals.us, according to the closer inspection of fermented corn stalk (Figure 7

Conclusion
In this study, fermented corn stalk (FCS) for biosorption of Cu(II) ions from aqueous solution was prepared.e optimal conditions were experimentally investigated as a solidliquid ratio of 1 : 5, pH of 5.5 with culture temperature of 28 °C, and 72 h fermentation time.Under the same biosorption conditions (biosorbent dosage of 0.15 g, natural pH, 100 rpm, 30 °C, and 30-minute reaction time), the biosorption capacity of fermented corn stalk (12.47 mg/g) was signi cantly greater than that of raw corn stalk (6.44 mg/g).SEM and FTIR demonstrated the modi cation of surface of the fermented corn stalk by Aspergillus niger treatment,  Advances in Materials Science and Engineering resulting in different biosorption capacity of the raw corn stalk.e abundant corn stalk adsorption present in renewable resources, such as biological, leads to high selectivity and adsorption capacity.Fermented corn stalk should be the focus on future efforts to develop effective bio-based adsorption materials.

Figure 6 :Figure 5 :
Figure 6: Biosorption capacity for Cu 2+ and binary metal ion mixtures with other kinds of divalent ions.
(b)), the Figure 7(c) shows the surface roughness of fermented corn stalk which clearly indicates the porosity on the irregular surface of corn stalk.

Table 1 :
Comparison of the biosorption of Cu 2+ by di erent dry modi ed materials.