Photocatalytic Treatment of Waste Water from Rice Husk Alkaline Hydrolysate

Olga D. Arefieva, Marina S. Vasilyeva, Ekaterina V.Ermolenko, Anastasiia V.Bychkova

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Photocatalytic Treatment of Waste Water from Rice Husk Alkaline Hydrolysate

Olga D. Arefieva1, 2, Marina S. Vasilyeva1, 2, Ekaterina V.Ermolenko3, Anastasiia V.Bychkova1

(Far Eastern Federal University, Vladivostok 690950, Russian Federation; Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences, Vladivostok 690022, Russian Federation; )

SUMMARY We carried out photocatalytic treatment of a lignin-free solution of rice husk alkaline hydrolysate [pH of 2, chemical oxygen demand (COD) of 12.8 g/L] using photo-Fenton process in the ultraviolet (UV, wavelength of 365 nm) and visible spectrum. Oxidation of lignin was performed at a constant mass ratio of COD:H2O2as 1:2 and Fe2+concentration of 80 mg/L. It was established that for the destruction of alkaline lignin, exposure to visible or UV radiation was necessary. Decolorization efficiency of a lignin-free solution depends on the dilution rate of the solution. The advanced oxidative photo-Fenton process (UV/Н2О2/Fe2+) made it possible to reduce the color of lignin-free solution at a dilution ratio of 1:5 by94% of decolorization efficiency, 87% of phenolic compound and 79% of COD. With the help of gas chromatography, the products of lignin destruction were low-toxic and usable.

In the recent years, there have been many proposals in the literature for involving non-wood plant raw materials (multi-tonnage agricultural crops of annual plants such as flax, kenaf, hemp, rice, oats and others) into the production of fibrous materials. Huge renewable waste generated in the production of rice cereals has long attracted the attention of researchers in different countries as a source of fibrous materials and a number of chemicals (Sarkar et al, 2017). But in the production of fibrous products, large volumes of waste water andalkaline hydrolysates, which contain mineral components, lignin, polysaccharides, low molecular weight resinous substances and others, are formed.

A number of studies showed that lignin-containing biomass (plant waste) can be processed by oxidative degradation in the presence of a multifunctional environmentally friendly catalyst based on Fe3+oxides(Amat et al, 2005; Xu et al, 2007; Sevimlia et al, 2014). As an oxidizing agent, it is possible to use Н2О2and/or air oxygen (Kasaikina et al, 2012; Lesin et al, 2012). Thus, oxidative processes using hydrogen peroxide and iron compounds are of interest for the treatment of lignin-containing waste water and processing of lignin-containing plant material (Catalkaya and Kargi, 2007; Lucas et al, 2012). The goal of this work is to choose the conditions for the effective oxidation of lignin in lignin-free solution of rice husk alkaline hydrolysates using photo-Fenton process in the ultraviolet (UV, wavelength of 365 nm) and visible (VIS) spectrum with the Fenton (in dark) process as the control (Pérez et al, 2002; Ahmaruzzaman and Gupta, 2011).

The husk of rice variety Krasnodar Krai was chose for the research. The experiments were carried out with a lignin-free solution that was obtained from the silica-free solution when the pH was changed to 2, while the alkaline lignin with a higher molecular weight precipitates, and the solution retains a low molecular weight soluble lignin [chemical oxygen demand (COD) of 12.8 g/L]. The process was carried out at a constant mass ratio of COD:H2O2as 1:2, and Fe2+concentration of 80 mg/L. Fig. 1 presents the decolorization efficiency of the lignin-free solution with a dilution ratio of 1:10 and 1:5. The results showed that at a higher dilution multiplicity of the solution (1:10), decolorization efficiency is lower, which is possibly connected with an increase in the pH value to 3 when the solution is diluted (Fig. 1). It is known that with increasing pH, alkaline lignin dissolves and the color of the initial solution increases (Minu et al, 2012). Using only Fenton’s reagent (experiment in the dark) leads to negative values of the decolorization efficiency (Fig. 1).

Fig. 1.Decolorization efficiency of lignin-free solution with a dilution ratio of 1:10 and 1:5 in photo-Fenton and Fenton conditions.

We observed the change in the optical density of the lignin-free solutions at different dilution rates when exposed to light for 10 d after irradiation in the UV region for 3 h. Fig. 2-A shows that after UV irradiation, decolorization efficiency sharply increases, but varies insignificantly in the visible spectrum between the dilution ratio of 1:10 and 1:5.And it reaches 86% at 10 d (240 h) with a dilution ratio of 1:10, and 94% at a dilution ratio of 1:5. Fig. 2-B shows kinetic curve of the decolorization efficiency change of the lignin-free solution in the UV and VIS spectrum for 3 h, and subsequent oxidation of lignin in the VIS spectrum for 10 d (240 h). For 3 h of irradiation, the decolorization efficiency in the UV spectrum increases, while in the VIS spectrum it has a negative value, which indicates the formation of organic substances with stronger chromophore groups in the solution as a result of the destruction of lignin. Further, the process of lignin destruction in the VIS spectrum proceeds quite intensively and on the 4th day (96 h), the kinetic curves of decolorization efficiency had no change in the UV and VIS spectrum. The decolorization efficiency of the lignin-free solution stabilizes and reaches 94%. The presence of UV or VIS radiation in the Fenton system made it possible to achieve a high degree of decolorization. The presence of the UV spectrum leads to an increase in the initial rate of the decomposition of alkaline lignin, apparently associated with the formation of more hydroxyl radicals. Evaluation of the destruction is also carried out by COD and phenolic equivalent (Fig. 2-C and -D).The kinetic curves of the change of COD and the content of phenolic compounds in the UV and VIS spectrum are similar.After 3 h of irradiation, the parameters are significantly reduced and the process continues steadily. On the 10th day (240 h), the removal efficiency of phenolic compounds from solution is 87%, while that of COD is 79%. Experiments in the dark have shown that the degree of decolonization reaches only 22% (Fig. 2-A and -B). COD values are practically unchanged (Fig. 2-C), while the content of phenolic compounds is reduced to 42.3% (Fig. 2-D). The results show that the oxidation of the alkaline lignin in dark practically does not occur, and it is therefore necessary to expose to the UV or VIS irradiation.

The color of the solution diluted five times, after oxidation in the UV and visible spectral spectrum, practically reaches the regulatory requirements, while the COD value of the solution does not meet the regulatory requirements (Table 1). However, undissolved organic substances remain in the solution, as evidenced by high values of COD solutions (550–540 mg/L).

Table 1. Characterization of lignin-free solution before and after oxidative degradation.

Table 2. Extract components identified by chromatographic-mass spectrometric analysis.

UV and VIS represent the ultraviolet (wavelength of 365 nm) and visible spectrum.

MPC, Maximum permissible concentration; ‘+’ means presence in extract; ‘–’ means absence in extract.

The composition of organic compounds after the destruction of lignin-free solution using the photo-Fenton process in the UV and VIS spectrum is almost the same. The resulting aqueous solution contains low molecular weight oxidation products of lignin-aromatic hydrocarbons and carboxylic acids (Table 2). The resulting low-molecular products are mainly low-toxic and have biological activity (antiseptic, insecticidal, fungicidal, antibiotic and other properties). The standards for reservoirs of cultural, domestic and domestic-drinking purposes were established for only three compounds.

It should be noted that the concentration of residual iron in solution exceeds the normative value for water objects for domestic and drinking purposes. Such waste water is not allowed to be discharged into water bodies or transferred to urban sewage systems (Vidal et al, 2001; Raj et al, 2007). Iron is a biologically active element, and it is necessary for plants to form chlorophyll (Khataee et al, 2014). Therefore, considering that biologically active organic compounds and iron cations remain in the solution formed after lignin oxidation, it can be used after additional studies.

Thus, this study showed the possibility of using the photo-Fenton process for the destruction of lignin in lignin-free solutions of rice husk alkaline hydrolysates. It was established that the decolorization efficiency of lignin-free solution depends on the dilution rate of the solution. At a dilution ratio 1:5 (COD:H2O2as 1:2), the decolorization efficiency was 94%. It is shown that the action of the VIS or UV irradiation is necessary to destroy alkaline lignin. Under photo-Fenton process (UV/Н2О2/Fe2+), the removal efficiency of phenolic compounds is 87%, and that of COD is 79%. With the help of gas chromatography, the products of lignin destruction, which are low-toxic, can be applied, for example, as antiseptic agents.

SUPPLEMENTAL DATA

The following material is available in the online version of this article at http://www.sciencedirect.com/science/journal/16726308; http://www.ricescience.org.

Supplemental File 1. Materials and methods used in this study.

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12 August 2018;

29 October 2018

Olga D. Arefieva (arefeva.od@dvfu.ru)

Copyright ? 2019, China National Rice Research Institute. Hosting by Elsevier B V

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer review under responsibility of China National Rice Research Institute

http://dx.doi.org/10.1016/j.rsci.2018.10.008

(Managing Editor: Wang Caihong)