Catalytic Reduction Performance of Carboxylated Alginic Acid Derivatives

In this study, the catalytic reduction behavior of carboxylated alginic acid derivatives has been investigated against the harmful organic dyes including Methyl Orange (MO) and Congo Red (CR). Alginic acid was rstly oxidized through an easy addition of KMnO 4 as an oxidizing agent. A carboxylated alginic acid (CAA) has been interacted with selected metal ions (Sn, Fe, Ni, and Zr) through coordination bonds at the value of pH = 4 to form the corresponding metal complexes namely: Sn-CAA, Fe-CAA, Ni-CAA and Zr-CAA. The consistency of the coordination was conrmed by several spectroscopic techniques including FT-IR, XRD, SEM, and EDX. The catalytic reduction of these metal ion-based products was carried out against MO and CR in the presence of NaBH 4 as a reducing agent under UV irradiation. All catalysts based metal complexes showed enhanced catalytic reduction against CR compared to MO. Among all those mentioned metal complexes Sn-CAA showed the best catalytic reduction of these dyes. The time taken by the Sn-CAA for CR, and MO is 5 and 7min respectively. Ni-CAA was classied as the second ecient product against both dyes, where the reduction process took 20 and 9 min respectively. The other two catalysts took a long time for CR and MO reduction. Zr-CAA showed more than 80 % reduction of only CR dye within 20 min. Whereas, Fe-CAA did not show any signicant sign of reduction against both the dyes after the same time. The order of higher catalytic reduction was illustrated as: Sn-CAA > Ni-CAA > Zr-CAA = Fe-CAA.


Introduction
Nowadays, the use of biopolymer as a catalyst is increased due to minimizing the procedure expense and make several applications more practicable and environmentally friendly. The biopolymers such as chitosan (CS) 1 ,starch 2 , alginate and cellulose 3 etc. have been extensively utilized as heterogeneous green catalysts in the chemical transformations. Furthermore, supramolecular designs of usual carbohydrates in recent times attracted much attention as "smart materials" from bio and nano engineering perspectives. These polymers contain various functionalities that exhibit good catalytic applications. When looking at the structure of alginic acid it comprises of homopolymeric blocks of 1-4linked β-D-mannuronic acid (M) and R-L-guluronic acid (G). Both the acidic units M and G were joined with areas of alternating structure in variable proportions of MG, GG and MM blocks. 4 Moreover, the eyecatching structures of AA are reactive carboxyl and hydroxyl functional groups. This feature made the composition as a template for synthesizing metal coordinate. The functionalization of the polymer material will rise its choosiness towards a speci c ion. Permissible to e ciently exploit the hydroxyl groups in the alginic acid an oxidation reaction was performed by means of potassium permanganate (KMnO 4 ), which alters the hydroxyl groups of AA into carboxyl groups namely carboxylated Alginic Acid (CAA) [5][6][7][8][9] . The choice of a biopolymer for instance AA has speci c applications like photo-luminesce, biocompatibility and conductivity, which is really essential, and could be utilized in place of different technological purposes. As a result of the existence of carboxylic and hydroxyl group in its main sequence it shows exceptional lms forming, membrane, bers, and gels abilities 10 . The higher valence metal ions amalgamated materials remain xated to prepare novel composite having respectable performances like removal and catalytic applications etc. 5 There are many types of synthetic dyes used yearly in millions of tons by the industries 11 . During the dyeing process, many color pigments released into the water reservoir from paper, garments, and pharmaceuticals industries etc. to make water polluted and toxic for the surrounding environment. The strong color of azo dyes is because in their chemical formula they contain (-N=N-) groups [12][13] . The azo compounds contain the biggest group of synthetic organic dyes and hugely utilized in food, paper, textile and cosmetic industries. Among them, MO has been widely utilized in laboratories as an indicator and in plastics, paper, textile, cosmetics, rubber, pharmaceuticals and printing industries [14][15] . The azo group present in MO lowers its biodegradability and makes it harmful to the ecosystem. It causes severe health problems like diarrhea, vomiting, and nausea 16 . For that reason, eliminating and reduction MO dye from water ecosystem is necessary. Previously, there were a lot of interest arises in hydrogenation of catalyst and heterogeneous catalysis [17][18] . Amongst the disclosed industries, fabric industry arises as a rst rank for coloration of bers, clothes etc. in using dye molecules. Even though, it's really tough to eliminate industrialized azo-dyes wastes from water and the utmost techniques utilized for treating the industrial e uents are catalysis, ion exchange, chemical coagulation, occulation, precipitation, adsorption, biological degradation, membrane ltration, and adsorption using active carbon treatment, ozonation, denitri cation process 19 electro-Fenton degradation 20 electrochemical sensor etc. Moreover, among them catalysis shows very good activity for reducing these dyes., As a continuation of our research for the fabrication of different types of hybrid nanocomposite materials for the eco-friendly applications, we giveaway easy and a simple process, i.e. aqueous phase preparation of M-CAA by means of natural stuffs such as seaweeds obtained as a low cost, eco-friendly in addition to reducing agents. The prime theme of our present work was to accomplish a catalytic effectiveness of natural products as a catalyst in the reduction of commercially available carcinogenic Congo Red (CR) and methyl orange (MO) as azo dyes from the aqueous solution functionalized and metal coordinated biomaterials as catalysts to reduce these harmful organic dyes.

Characterization
All spectrophotometric measurements recorded with UV-visible spectrophotometer (Shimadzu 1800) with a 10 mm (path width) quartz cell. Orion pH meter (model EA 940) was employed for the test solutions and pH measurements and a digital micropipette (Volac) for the preparation all solution. Deionized water obtained from Milli-Q Plus system (Millipore, Bedford, MA, USA) and used for the preparation of solutions. eld emission scanning electron microscope (FESEM) (JSM-7600 F, USA) for morphological studies also for elemental studies were carried out with energy dispersive x-ray spectroscope (EDX) tted to FESEM and x-ray photoelectron spectroscope. Fourier transform infrared (FTIR) spectroscope (Thermo scienti c, USA) for collection of FTIR spectra data (Range: 500-4000 cm −1 ).

Preparation of coordinated M -CAA
For oxidizing alginic acid (5g), it was treated with an oxidizing agent potassium permanganate having concentration of 10mM. The process was done under strong mechanical strring for 30 min at temperature range of 45°C. Due to the explosive nature of potassium permanganate, it was used within the range of 10mM as reported earlier 12 . The polymer solution goes under centrifugation to obtained CAA. The material was thoroughly washed with distilled water to reach the pH up to 6-7. The CAA dried in oven for 2h at 60°C. To prepared the metal coordinates of CAA with Ni +2 , Zr +4 , Sn +2 , and Fe +2 metals, two round bottom asks were taken and each one contain 2g CAA in 100ml DI water. It was than treated with

FT-IR Fourier-transform infrared spectroscopy
After doing oxidation of the AA, we checked the formation of carboxyl group by FTIR. The peak at around 1745cm -1 shown in Fig. (1b) is coming from carbonyl group present in AA after oxidation. At around 3000-3500cm -1 we can clearly observed the fall in intensity of peak in g. (1b) from the parent material i.e. AA Fig. (1a) which con rms the formation of the carbonyl group to AA. During the meantime we also treat the CAA with metals as well and to check the formation of the coordinate bond with metals. When the coordinate bond formed between metal and CAA the intensity at around 1745cm -1 disappeared Fig. (c-f), and again it shows broad peak at around 3000-3500cm -1 .This approves that protons that are in the carboxylic acid had changed and formed a harmonic bond with metals 31-32 .

The X-ray diffraction (XRD) of CAA and M-CAA
To carry out X-ray diffraction pattern for studying the lattice structure of CAA and M-CAA adsorption materials. Fig. 2 shows the XRD spectra of CAA (a), Zr-CAA (b), Sn-CAA(c) and (d) Ni-CAA respectively.
The distinguished crystalline peaks of CAA(a) were located at 14.47° and 21.31°, respectively, representing the re ections of the (422) and (511) planes Fig (2a) 33 . The XRD pattern analysis of Zr- CAA   Fig (2b) showed that there was a tetragonal phase of Sn and Ni Fig (a-d). The analysis also con rmed the semi-crystalline nature, which improved the structural stability of the manufactured M-CAA adsorbent material.
3.3. Scanning electron microscopy (SEM) image and EDAX for CAA, Sn +2 and Zr +4 Ni +2 , Fe +2 By checking the FE-SEM pictures shown in Fig 3, the morphology of net CAA and Zr-CAA as a selected example can be clari ed. The SEM images a, b represents the CAA surface which consists of aggregated tiny particles in both lower and medium magni cations which is in accordance with the literature 34 . The images from (c, d) represent from medium to high magni cations of Zr-CAA (x =7,500 and 60,000) respectively. The medium magni cation shows the surface consists of sponge like shape; while the resultant high magni cation image (x = 60,000) shows accumulative tiny spherical particles in the nanoscale. The complex mechanism between the selected metal cation and CAA can be used to explain the difference in morphological structure formed in these imaged compare to the reported images for pure CAA 34 . These tiny points indicate Zr are effective in coordination with CAA. The Zr-CAA images give a good evidence for the complexation process throughout metal ion chelation. However, the reported CAA images undoubtedly demonstrates a complete different morphological features than that observed in this text. Furthermore, the synthesized Zr-CAA selected sample has been further investigated by EDS analyses to quantitatively de ned the elemental composition. The Zr were undoubtedly spotted in Zr-CAA formulation. Zr alongside C and O detected in Zr-CAA, proposing the effective formation of Zr-CAA. C and O elements were detected due to the presence of carboxylic alginic acid. Zr was identi ed along with C, and O in Zr-CAA catalyst and the weight percentage for C, O and Zr in the form of Zr-CAA is 54, 44 and 2% respectively.

Catalytic study
Due to the diverse application of dyes and their unhealthy effect on the surrounding environment and aquatic life, we select two anionic dyes MO and CR for our study. We prepared a facile method for the reduction of these dyes. The CAA metal coordinate complex utilized as a catalyst in the presence of a reducing agent using UV-vis spectrometer technique in the wavelength range of 200-800 nm. To know the catalytic activity of the catalyst against model reaction of CR and MO we prepared fresh sodium borohydride having concentration of 0.1M. We took 2.5ml of dyes in cuvette, take the reading after that we add 0.5ml of reducing agent sodium borohydride, and measured it. Then 10mg of metal catalyst immersed in a cuvette and MO at 468 nm and CR at 495 nm. The time interval for the reaction was 1min.
The calculation of reduction of these dyes done according to time and percentage reduction equation as follows, Where A 0 is the UV-vis absorption intensity of dyes at time t=0, and A is the UV-vis absorption of same solution at time t (min). C 0 is initial concentration of the dyes and C is the concentration after time' t'.
Dyes reduction was carried out according to the proposed mechanism, as Figure 4

Evaluation of M-CAA toward CR and MO
The selection of materials and their nature and chemical properties are important for many different applications, especially water puri cation. Therefore, it is necessary to choose materials with unique properties in terms of e ciency in their work as well as their cost and have an anti-bacterial feature. Many applications and researches applied to water puri cation. Nevertheless, all methods have a cost and may require high-cost devices, which makes their e ciency in the end less than Less than hoped for in the process of purifying the water from toxic material such as azo dyes. The method of catalytic reduction of dyes is a simple and easy method. In addition, many nanoscale materials used to remove dyes or even reduce them, but there are some disadvantages to them, which is the di culty of removing them from the water, which requires di cult techniques or even ltering it to get rid of them. All these are defects of some of the materials mentioned. polysccride such as chitosan or carboxymethylcellulose.
They have been used to reduction dyes in many literatures because of the properties that make it easy to get rid of them after use.in this study, in this work The carboxylated alginic acid CAA used as a safe organic biopolymer that contains many active groups that are important in interaction with metals as coordinate bonding for reduction harmful anionic azo dyes. Through the experiment in Fig 4a and 7a we found that AA and (NaBH 4 ) without modi cation does not more effect on ionic dyes because they both carry the same electrical charge also, when CAA and (NaBH 4 ). There were more effective groups such as the carbonyl group and hydroxyl, but in the process of studying it with harmful dyes such as M.O and C.R, the result was unsatisfactory due to the presence of high electrical charges on many hydroxyl and carbonyl groups as Fig 4b and 7b. Similar to the charge carried by the dyes thus there was no signi cant effect on the reduction reaction.

Catalytic reduction of CR
An anionic diazo dye, which is widely utilized in various industries, is Congo red (CR). Because of its utilization in various elds like textile, cotton, silk, food, paper and pulp manufacturing factories, CR is very important in various azo dyes. It's utilization in medical eld as a biological stain for the identi cation of amyloidosis, and as an pointer in acidic media [35][36] . In addition to the extensive uses of CR, it also has toxic effects on wildlife. CR can cause vomiting, gastrointestinal irritation, skin and eye irritation, diarrhea, etc. In women it causes liver tumors for exposing long-term to CR [37][38] . CR could report allergic reaction. The CR disposal rate is extraordinary in the use and manufacturing procedure. Traditional treatments cannot effectively remove CR. In addition, under anaerobic conditions, CR digested into human carcinogen called benzidine [38][39] .
The reduction of CR takes place in the existence of M-CAA and NaBH 4 . For reducing, 2.5 ml of 0.13 mM CR solution was placed in a quartz cuvette and add 0.5 ml of NaBH 4 into the solution. CR shows two strong absorption peaks in the UV-visible spectrum at 493 and 344 nm. In the absence of a catalyst, the peak intensity does not change for AA as shown in Fig 5a. We than modify the AA to CAA and again check the UV for CR it also shows similar result like its predecessor Fig. 5b. After that, we make coordinate complex of CAA with metals like Sn, Fe, Zr and Ni (M-CAA).
By adding 10 mg of M-CAA, the peak intensities at 493 and 344 nm begin to decline. As shown in Fig 6a-d, the decrease in CR was recorded by measuring its UV-vis spectrum at regular time interval of 1 min between the two measurements. Adding of M-CAA reduces both peak intensities. Though, as the intensity increases, a fresh peak arises at 252 nm, as displayed in Fig (6a, c, d). Therefore, the steady vanishing of peaks at 493 and 344 nm is because of CR reduction. Therefore, the arise of a new peak at 252 nm is due to the formation of free amino group compounds in the course of the reduction process of CR. The reduction CR was done against different metal catalyst like  40 . Thus, the overall reduction process of the CR monitors the pseudo-rst order kinetics. The calculation of the rate constant (K app ) was based on the relationship between ln(A t /A 0 ) and the reduction time (as shown in Fig (7b) by formula (1). The calculation done for K app from the slope of CR existed as K app = 0.6602 for Sn and K app = 0.427 of Ni and K app = 0.173. Zr and K app = 0.173 for Fe.

Catalytic reduction of MO
With the growth of industry, human beings are facing severe environmental problems because of releasing enormous number of pollutants into the water-based environment, particularly dyes and other organic molecules. The ejection of sewage into the ecological unit is a chief hazard to marine ecosystem and human health. The presence of azo (-N=N-) group in chemical structure of the dyes gives them deep color [12][13] . Amongst organic synthetic dyes, azo group has established a major class of dyes and is extensively exploited by means of textile, food, cosmetic, paper and pulp manufacturing factories. MO is also anionic azo dye often utilized as an indicting material in laboratories and in the paper, textile, cosmetics, rubber and pharmaceutical industries etc [14][15] .
Emission of azo dye unwanted liquid from numerous industries, exclusively the textile industry, into the aquatic ecosystem pose a thoughtful warning. Therefore, the improvement of fresh methods intended for the decomposition of diazo has attracted boundless attention. The MO has intense peak around 468 nm. According to earlier reports, NaBH 4 alone can reduce MO dye thermodynamically which is bene cial but the reaction is very gentle kinetically 41 . Lack of an appropriate catalyst, even within a few hours, the peak intensity at 468 nm peak of MO remained constant [42][43] .
MO was strong absorption peak in the UV-visible spectrum at 468 nm. In the absence of a catalyst, the peak intensity does not change for AA as shown in Fig 8a. We than modify the AA to CAA and again check the UV for MO it also shows similar result like its predecessor Fig. 8b. In order to catalytically reduce MO, rst put 2.5ml of dye solution into the quartz cuvette and then add 0.5ml of NaBH 4 having 0.1M concentration. The UV-vis spectrum of MO shows an intense peak at around 468 nm. Before adding the catalyst, the peak intensity of MO remains intact at 468nm. Though, after introducing 10mg of metal catalyst, the peak intensity at around 468nm began to decrease.
As shown in Fig 9(a-d), the catalytic study of M-CAA was measured by assessing the UV-vis spectrum using a time span of 1 min. By adding M-CAA, the decrease in intensity of peak at 468 nm was observed. Therefore, the steady vanishing of the peak around 468 nm because of reducing the MO, usually in the existence of a vigorous catalytic agent, NaBH 4 reduces MO molecules at the azo spot by producing low molecular free amino group compounds. Therefore, during MO reduction process, the formation of free amino compounds is the reason for the creation of a new peak at 250 nm.
The effectiveness of (M-CAA) has been tested with the aforementioned metals. Below we see the gures for each metals and the extent of its effect on the dyes M.O, and we conclude from them that the Sn is the best metal, as we see that it did not take much time less than 7 min in the process of reducing MO dye compared to the rest of the metals (Ni, Fe, Zr) which take more time in Ni cause 20 min but for Fe and Zr not more effect as Fig 9a, d. The apparent rate constant (K app ) calculation was based on the relationship between ln(A t /A 0 ) and the reduction time as shown in Fig 10 by formula (1). The calculation done for K app from the slope of MO was K app = 1.44 for Sn and K app = 0.0413 of Ni and K app = 0.14. Zr and K app = 0.139 for Fe.

Catalytic reduction of mixed solution of M.O and CR with Sn-CAA
Through the previous results, we obtained detailed information about the two dyes through Sn-CAA. We mixed the two dyes with 10ml of 0.1mM of each CR and MO solution, and then added 2.5ml of the mixed in the UV cuvette, (0.1 M) 0.5 ml NaBH 4 and 10mg of Sn-CAA for analysis. Under the same conditions of testing the two dyes separately, a decrease in the absorption of the UV spectrum is observed every minute. So, when the two dyes are mixed, we have one peak, and it is broad and large, which indicates that the two peaks overlap because they are both close to aggregate absorption (465 nm and 495 nm MO and CR, respectively. When the two dyes are added with Sn-CAA, they will affect the reduction process in a short time of less than about 6 minutes. We can see that the peak intensity decreases at 476 and 344 nm, and with increasing, at 252 nm A new peak appears with very high intensity, because free amino compounds are formed during the reduction of CR and MO, which shows the effectiveness of Sn-CAA in the reduction process, and even the two dyes together show excellent performance, as shown in Fig  11. The apparent rate constant (K app ) calculation was based on the relationship between ln (A t /A 0 ) and the reduction time (as shown in Fig 11(c) by formula (1). The K app measured from the slope of CR was K app = 0.598.

Recyclability
The recyclability of Sn-CAA as a coordination bond was determined. Following the same experimental procedure as before to reduce C.R and M.O. We used the same carboxylated Sn-CAA, washed with water, and reused in seven reductions of C.R and M.O solutions. In the course of seven uses of coordinated carboxylated alginic acid, a reduction of more than 90% was observed. In Fig 12 you can see the time taken to reduce the CR and MO solutions to more than 90% in the rst, second, third, fourth, fth, sixth, and seventh cycles. For the MO solution, in the 90% reduction process, the reduction was completed in less than 7 minutes. The rst cycle takes 7 while 9, 11, 13, 13, and 15 minutes for the other cycles. In the case of C.R, 90% of the reduction reaction can be completed in 5 minutes, while most of the conversion in the repeated cycle is completed in 5, 7, 10, 11, and 11 minutes. We have observed that the reduction time increases when using the same Sn-CAA, which indicates a decrease in catalytic performance. Furthermore, a comparison table regarding the using of different matrices against the reduction of CR and MO at 25 °C has been listed in Table 1.

Conclusion
Safe, low costly and ecofriendly modi ed alginic acid derivatives in the form of M-CAA were successfully prepared as coordinate materials including different metal ions (Sn, Fe, Ni and Zr). The desired materials were prepared by easy method using two steps. The nal products were con rmed by common characterization techniques including FT-IR, XRD, SEM and EDX analyses. These materials were applied as moderate new materials for the catalytic reduction of both (M.O, C.R) dyes. Sn-CAA showed the best catalytic reduction for CR and MO (5 and 7min) respectively. Whereas, Fe-CAA and Zr-CAA showed the longest catalytic reduction time against the same dyes. The optimum conditions for all the used catalysts were measured. Sn-CAA catalytic reduction showed an e cient recyclable product after 4 cycles then gradually become less e cient. Furthermore, Sn-CAA showed an excellent result for purifying natural water from toxic organic wastes as an example for real test sample.