Cr(III) removal from synthetic and industrial wastewaters by using co-gasification chars of rice waste streams
Graphical abstract
Introduction
Rice is the second most produced cereal in the world after maize. In 2017, 756.7 million tonnes of paddy rice were produced worldwide (FAO, 2017a). Portugal is the main rice consumer in Europe and the fifth larger producer after Italy, Russia, Spain and Greece, with an average annual production around 170,000 tonnes (FAO, 2017b).
The main wastes generated in rice production are rice husk (RH), rice straw (RS) and plastics; the latter are mainly composed by polyethylene (PE) from packaging of seeds and fertilizers. About 23% w/w of the total paddy rice is composed by RH (Prasara-A and Gheewala, 2017), and each kilogram of the harvested grain produces 1.0–1.5 kg of RS (Sangon et al., 2018). The current final destinations for these wastes are distinct: RH is used as bed material in poultry breeding, or as raw material for animal feeding; RS is incorporated in the soil or burnt in open-air at rice fields. In recent years, many studies have been conducted to find new valorisation routes for these wastes, such as (i) RS use in bio-refineries for different fuel and chemical production (Abraham et al., 2016), paper making (Kaur et al., 2017), or bioethanol production (Singh et al., 2016), (ii) RH use as building material, SiO2 source, or adsorbent (Liu et al., 2016), and (iii) PE recycling in new green routes (Al-Sabagh et al., 2016).
Gasification is a thermal technology that converts wastes into energy and carbon materials (Alauddin et al., 2010, Ramos et al., 2018, Watson et al., 2018). The main aim of gasification technology is to convert the wastes into syngas, which can be used as energy source (Kumar et al., 2009). However, a solid by-product named gasification char is also produced in this thermal technology, which unlike pyrolysis char has not yet been studied very often for possible valorisation routes (Fryda and Visser, 2015, You et al., 2017).
One of the most promising pathways for gasification chars is the use as adsorbent of pollutants from aqueous effluents (Benedetti et al., 2017, Galhetas et al., 2014a, Galhetas et al., 2014b), as they may have interesting porous structures for pollutant adsorption. Another interesting feature of these chars is the presence of mineral species or functional groups on their surfaces, which can play an important role in pollutant removal. In addition, these carbon materials have significant lower costs and higher environmental benefits when compared to the traditional commercial adsorbents, such as activated carbons, particularly if bio-wastes are used as raw materials in the gasification process.
To authors’ knowledge, there are few studies on the use of gasification chars from rice waste streams as adsorbents of pollutants from aqueous media (Phihusut and Chantharat, 2017, Prasara-A and Gheewala, 2017). The only published work on chars from co-gasification of rice waste streams as adsorbent materials was performed by the same team of this manuscript (Godinho et al., 2017).
Chromium (Cr) is a problematic pollutant in water, but also a valuable raw material for many industries (metallurgy, leather tanning, wood preservatives, chemical sector, among others). The European Commission (EC) recognised this fact by publishing, in 2014, a list of critical raw materials (CRM), regarding their supply risk and economic importance. Cr was included in this list (EC, 2014). Although Cr was removed from the revised CRM list in 2017 (EC, 2017) as its supply risk decreased slightly, it still is a raw material with high economic score.
In order to address some of the challenges referred above, a national project named “Ricevalor” was developed through the involvement of two research institutions (LNEG and FCT-UNL) and one Portuguese Company (Orivárzea) that produces, processes and sells rice in the national market. Under this project, rice waste streams were submitted to co-gasification for syngas generation; syngas was converted into energy carriers and raw materials for chemical industry (André et al., 2014, Pinto et al., 2016, Pinto et al., 2015). Gasification chars were valorised as adsorbents of valuable metals from aqueous media.
The main aim of this study was to use co-gasification chars from rice waste streams in the removal of Cr(III) from both a synthetic solution and an industrial wastewater. A Commercial Activated Carbon (CAC) was used for comparison purposes.
Section snippets
Origin and characterization of precursors
Samples of RS, RH and PE were supplied by Orivárzea Company. This company has rice fields located in Ribatejo region (Portugal). Additional data about the origin and characteristics of these precursors are available in Dias et al. (2017).
Gasification assays and origin of chars
The gasification assays were performed in a bubbling fluidized bed gasifier (0.08 m internal diameter × 1.5 m height) filled up with fine silica sand as bed material (Pinto et al., 2016). The feedstock flow rate was of 5 g min−1 (daf). The
Characterization of adsorbents
The properties of the adsorbent materials are conditioned by the properties of the biological precursors (RS and RH). The chemical composition of these precursors varies upon time; therefore, the properties shown in the following sub-sections are related with the chemical properties of the precursors as they were used in this work (Dias et al., 2017).
Conclusions
The chars produced in the gasification assays consisted mostly on ashes (mainly Si) and were characterized as non-porous materials.
For the removal assays in the synthetic solution, the chars always presented higher removal efficiencies and uptake capacities than CAC, either by precipitation or adsorption. For the removal assays in the industrial wastewater, G2C-5 presented better results than CAC when precipitation occurred, but CAC performed better when adsorption ruled.
The adsorption
Acknowledgments
This research was funded by FEDER through the Operational Program for Competitive Factors of COMPETE and by Portuguese funds through FCT (Foundation for Science and Technology) through the project PTDC/AAG-REC/3477/2012 – RICEVALOR “Energetic valorisation of wastes obtained during rice production in Portugal”, FCOMP-01-0124-FEDER-027827, a project sponsored by FCT/MTCES, QREN, COMPETE and FEDER.
The authors also acknowledge the Foundation for Science and Technology for funding Maria Bernardo’s
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