Extremely fast and efficient methylene blue adsorption using eco-friendly cork and paper waste-based activated carbon adsorbents
Graphical abstract
Introduction
Annually industrial dyeing processes release between 30,000 and 150,000 t of dyes into wastewaters (Anjaneya et al., 2013), raising serious environmental concerns due to the toxicity and carcinogenic properties of most of these dyes. This further intensifies clean water scarcity, which is already seen as one of the most pressing concerns of our society (World Economic Forum, 2016). Wastewater treatment is, therefore, of the utmost importance, and may provide a very important source of clean drinking water. Various methods have been used to remove dyes from wastewaters. Nevertheless, adsorption is considered the most effective and simple method for water decontamination (Rafatullah et al., 2010).
This technique does not generate harmful intermediate products, this being a major advantage over other techniques such as photodegradation/photocatalysis. These features explain the great number of investigations addressing adsorption (Arabi et al., 2017) in comparison with other techniques. Activated carbons present singular properties, such as their high specific surface area (SSA) and adsorption capacity, and as such are one of the most commonly used adsorbents. Nevertheless, their high production cost hinders their widespread use (Rafatullah et al., 2010). Waste materials may be an inexpensive source of activated carbons which could allow the spread of this technology. Activated carbons have been prepared using a wide variety of waste, such as spent coffee grounds (Ma and Ouyang, 2013), waste carbon (Cheng et al., 2017), ficus carica bast (Pathania et al., 2017), palm shell (Wong et al., 2016), among others.
Cork is a renewable and sustainable material extracted from the outer bark of the cork oak tree (Quercus Suber L.), and Portugal is the world's largest cork producer (∼50%) (APCOR, 2016). Cork is fully sustainable, as the tree is not harmed during harvesting every 10 years, living on as a carbon sink for 200–300 years, and the equivalent of any CO2 realised from the cork during processing will be absorbed by the tree to produce the next growth of bark. This is a very relevant advantage over other carbonaceous materials contributing towards cleaner production. Cork has singular properties such as exceptional thermal, acoustic and vibrational insulation, which allows its use in applications ranging from construction and transports to sports. Nevertheless, the main application for cork is as wine stoppers. Cork powder is the main waste of the cork industry, being generated throughout the fabrication stages of various cork products (Atanes et al., 2012). In the granulation process, for manufacturing black and white agglomerated cork construction products and granules, around 35% of cork powder is produced (Demertzi et al., 2016), with an annual production estimated to reach around 50,000 t (Cardoso et al., 2008). This residue has low economic value, and for that reason it has been mainly used for energy generation through its combustion (Demertzi et al., 2016). However, recent studies have demonstrated the possibility of using cork waste as a precursor in the preparation of activated carbons. Cabrita et al. (2010) studied the removal of acetaminophen (an analgesic) from water, while Cardoso et al. (2008) evaluated the feasibility of using cork-based activated carbons to control atmospheric pollution. This exciting strategy may attribute value to this industrial waste. Cork's unique microstructure, consisting of hollow polyhedral cells (∼20 μm diameter) and containing up to 200 million cells per cm3 (Pereira, 2007), could lead to very high surface area materials and low water content (energy saving) (Aroso et al., 2017), these being crucial advantages over other carbonaceous materials. Waste cork-based activated carbons may be a low-cost and sustainable alternative to the use of commercial activated carbons. This could be particularly relevant to the Portuguese context, since 34% of worldwide cork oak forest is located in Portugal (Gil, 2014), responsible for 50% of global cork production. Cork-based activated carbons have been used as adsorbents to remove volatile organic compounds (Cardoso et al., 2008) and emergent pollutants (Mestre et al., 2014).
However, despite their promising results, commercial alkaline activators such as KOH or K2CO3 were used in these previous investigations to prepare the activated carbons. These additives are detrimental to the activated carbon production cost and carbon footprint. In the little literature which exists addressing the synthesis of cork-based activated carbons, only KOH (Atanes et al., 2012), K2CO3 (Carvalho et al., 2004) and phosphoric acid (Aroso et al., 2017) have been used as activation agents. Here, an innovative and more eco-friendly strategy was implemented, in which a mixture of an alkaline industrial wastewater (coming from the local Portuguese pulp and paper industry) and sodium hydroxide was used to produce cork-based activated carbons. This is the first ever report of the production of cork-based activated carbons using an alkaline waste for the chemical activation of the cork. The use of this industrial wastewater instead of virgin raw materials (e.g. sodium hydroxide) may reduce the activated carbons production cost and carbon footprint in comparison with those produced solely using commercial activating agents, while simultaneously mitigating the amount of wastewater disposal from the paper industry.
The adsorbents prepared were then used to extract methylene blue (MB) from synthetic wastewaters, which to the best of our knowledge has never been reported for cork-based activated carbons. MB is a well-known monovalent cationic synthetic dye used in different fields (e.g. medicine, chemistry and industry), which is known to be toxic to humans (Novais et al., 2018) as well as the environment, and, therefore, its presence in wastewaters is highly undesirable, and its removal mandatory (Rafatullah et al., 2010).
In this investigation, cork-based activated carbons exhibiting very high surface area (∼1670 m2/g) were first produced using a waste-based activator, and then evaluated as MB adsorbent material. The influence of MB initial concentration, contact time and adsorbent concentration on the MB removal efficiency and uptake by the cork-based activated carbon was studied. Results demonstrate the feasibility of using significant amounts of an unexplored industrial alkaline waste (50 vol%) to replace commercial activators (e.g. sodium hydroxide) in the production of eco-friendly and sustainable cork-based activated carbons, showing very high and fast MB removal capacity. This innovative strategy reduces the activated carbon production cost in comparison with those produced using solely commercial chemicals (the alkaline wastewater has currently no commercial value, being considered a waste) while simultaneously attributing value to two distinct waste streams (wastewater effluent and cork residues), and mitigating the environmental impacts associated with the wastes disposal.
Section snippets
Materials
The cork powder waste was supplied by a Portuguese cork industry (Amorim). This waste, produced during the cork granulation process, has a bulk density of 50–70 kg/m3, and the particles are in the range of ∼0.5–1 mm in dimension.
The cork activation was performed using a mixture of 10 M sodium hydroxide (ACS reagent, 97%; Sigma Aldrich) and alkaline wastewater (effluent; pH ∼ 11.5) generated during the Kraft process in a Portuguese pulp and paper plant. The activators were used in a 50:50 vol%
Precursors characterisation
A photograph and SEM micrographs of the cork powder waste are presented in Fig. 1. The cork powder is composed of irregular particles, sized between 400 μm and 1 mm, showing low specific surface area (8.2 m2/g) (N2 adsorption-desorption isotherms at 77 K presented in Fig. 2a) which is attributed to their closed cells. Untreated cork has very low open porosity, this being one of the reasons why this unique material floats in water and has low permeability. In line with this remark, the micropore
Conclusions
In this work, and for the first time, the feasibility of using an alkaline industrial wastewater (pH ∼ 11.5) as a partial replacement for commercial activating solutions in the synthesis of cork-based activated carbons was investigated. Cork residues have been successfully activated by using a mixture of sodium hydroxide and alkaline wastewater (50 vol%), leading to the production of mesoporous activated carbon with very high specific surface area (1670 m2/g), surpassing all previous
Acknowledgements
This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. R.C. Pullar wishes to thank FCT grant IF/00681/2015 for supporting this work, and R.M. Novais wishes to thank FCT project H2CORK (PTDC/CTM-ENE/6762/2014).
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