The generation of waste from urban and industrial activities has achieved unsustainable levels, occupying an important place in the planning of cities, as well as in managing industrial processes (Belaud et al., 2019). Therefore, the development of closed-loop technologies has emerged as a contemporary way to convert trash into treasure, even though there are cultural barriers to be overcome (Russo et al., 2019).
For instance, proposing useful applications for the sludge produced in sewage and water treatment plants is one of the hardest tasks for public managers. For comparative purposes, it is estimated that the United States produces more than 6.5 million metric tonnes of sewage sludge per year (Venkatesan et al., 2015), whereas in Brazil, where only around 59% of municipal solid waste is appropriately disposed of in sanitary landfills, the production can achieve up to 3.0 million tonnes per year (Alfaia et al., 2017; ANA, 2017). This kind of biowaste is usually discarded in landfills, causing a high environmental impact not only because of the large production worldwide but also for being considered a hazardous material due to its metal and pathogenic microorganisms’ content (Jafari and Botte, 2021). Additionally, unless the distance to be covered for sewage sludge disposal exceeds a certain limit, the relatively low cost of fee taxes has discouraged the implementation of cleaner and more profitable technologies (Tesfamariam et al., 2020).
Fortunately, several alternative strategies have been proposed in order to mitigate the environmental impact of sewage sludge as well as explore its continuous and growing production, such as its conversion into biofertilizers and its use as a source of bioenergy, two sustainable applications that have gained ground in the world scenario (Ding et al., 2016; Zaharioiu et al., 2021).
Concurrently, an industrial waste which has attracted the attention of managers is the one from lead-acid batteries. Despite the well-established lead-acid batteries recycling technologies, a recent study highlights that, unlike lead and plastic casings, the electrolyte sulfuric acid, which corresponds to up to 44 wt.% of the electrolytic cell, finds a limited route when being converted into CaSO4 or Na2SO4 (Ballantyne et al., 2018). The spent sulfuric acid (SSA) presents a concentration ranging from 10 to 25 wt.%, demanding a large amount of base to be neutralized, as well as several heavy metals, such as Fe, Zn, Cd, Sb and Pb (Zhao et al., 2021). The multiple-valence iron is considered to generate fatal effects to the battery capacity, leading to corrosion of the electrodes, which in turn causes loss of active materials, as well as promotes selective discharge, undesirably releasing O2 and H2 gases. The resulting effect of iron contamination can be understood as a decrease in the battery life cycle (Lam et al., 2021; Liu et al., 2011).
In view of the foregoing, some technologies have been evaluated in order to remove iron ions from SSA aiming at enhancing the lead battery recycling process. Qifeng et al. (2016) reported a successful method based on solvent extraction followed by stripping in which the selectivity for iron(III) achieved 99.9%. Despite the excellent result, the employed method requires the use of an organic solvent as the extraction agent, which may be characterized by a critical shortcoming for industrial application, and a stage with high potential for the generation of new environmental liabilities.
Alternatively, solid-liquid adsorption is considered a powerful technique for the removal of chemical species from solution. However, the strong acid medium of the SSA demands a high chemical stability on the part of the solid adsorbent, since the drastic conditions favor its dissolution. For instance, among the adsorbents largely used in industry, zeolites show a vulnerable chemical structure, susceptible to dissolution in extreme pH conditions (Ennaert et al., 2016; Hartman and Fogler, 2007). This is an important disadvantage and makes the use of zeolites for direct SSA purification unfeasible, mainly for type A zeolites, which have a Si:Al ratio of 1 and present interconnected cavities, a material highly susceptible to dissolution in strong acidic medium (Julbe and Drobek, 2016).
On the other hand, carbon-based materials present a superior chemical stability, given the extremely drastic activation conditions that an ordinary charcoal is submitted to in order to have its surface modified. Typically, such conditions include mineral acids or bases at 10-20 wt.%, followed by thermal treatment at temperatures that achieve up to 400 or 500 ºC (Seo-Hyun et al. 2016).
In regard to carbon-based material production, pyrolysis is the most common method employed. Although pyrolysis is a well-known technology for the conversion of biomass into charcoal, the inert atmosphere, high temperature, and numerous other factors are significant disadvantages, depending on the specific type of route chosen, e.g. fixed or fluidized bed (Lopez et al., 2017).
Hydrothermal carbonization (HTC) emerges as an alternative technology to pyrolysis, mainly due to the milder experimental conditions required for biomass conversion. HTC is a thermochemical-based technique capable of converting biomass into solid, liquid and gas phases without the need for its pre-drying, which provides an interesting advantage in energetic terms (Niinipuu et al., 2020). Product distribution fundamentally depends on the type of feedstock and the temperature of the process (usually set to 180-250°C). However, there is also dependence on the reaction time and on the water:biomass ratio (Nizamuddin et al. 2017).
Also, HTC does not require any inert atmosphere to take place, since the mechanism of the process contemplates hydrolysis of the cellulose, decomposition of the hemicellulose, dehydroxylation, decarboxylation and dehydration reactions practically insensitive to the presence of O2 (Bevan et al., 2020). Usually, sources of biomass rich in lignocellulose are widely employed in the HTC process, however, sewage sludge has been considered as a welcome feedstock because of its usually high content of water. Several works have reported the conversion of sewage sludge into hydrochars, also named biochars due to its environmental appeal (Bolognesi et al., 2021; Chen et al., 2020). Additionally, the compositional heterogeneity displayed by sewage sludges, such as the blend of organic and mineral materials may favor the formation of biochars with more appropriate surfaces to be employed in sorption processes.
The inter-loop concept represents several approaches extensively explored in the context of the circular economy by the global industry with regard to feeding one process by using tailings from another, similar to the open-loop strategy present in the well-established reuse of blast slag from steel melting furnaces as an additive for Portland cement production (Tsakiridis et al., 2008), and numerous proposals reported in the literature which involve a myriad of possibilities (Gadipelly et al., 2014; Han et al., 2021). However, these methods usually do not predict a continuous reuse of such tailings in order to create interconnected new loops, which would purposefully develop and strengthen other industrial processes.
This work reports an approach involving a first recycling process, characterized by the hydrothermal carbonization of a biowaste into a valuable material, in order to apply it in a second recycling process, related to the reuse of spent sulfuric acid from lead-acid batteries, aiming at synergism between the two processes.