Statement of Novelty

There have been various reports on the use of biochar for assisting in the adsorption of Phcs, however, reports on the utilization of biochar for improving Phcs removal during the treatment of wastewater at WWTP are limited. This synopsis aims to bridge that gap by specifically presenting the possibility of improving the removal of Phcs during wastewater treatment using biochar, with emphasis on diclofenac (DF), ibuprofen (IB), and metformin (MF) removal. Furthermore, Estonia is chosen as a case study owing to the limited research on biochar and its application within this region.

Introduction

Pharmaceuticals (Phcs) and Their Impact on the Environment

Pharmaceuticals (Phcs) are modified biologically active substance used to cure or prevent ailments in animals and humans [1, 2]. It includes analgesics, antibacterials, as well as antiepileptics [1, 3, 4]. Reportedly, the use of Phcs in the year 2020 was about 4.5 trillion dosages [5] and could increase in the future [6]. However, concerns about their presence in the environment and threat to aquatic lives have been raised by several authors [6,7,8,9,10,11]. The threats they pose to the environment include (1) The development of pathogens that are more resistant to treatment and threaten the health of lives within the environment [12,13,14] (2) Their bioaccumulation is poisonous to the environment [15] and (3) Their presence impacts the food web within the aquatic eco-system in an unfavorable manner [16, 17]. Reportedly, two of the major sources of Phcs within the environment is from the effluent of wastewater treatment plants (WWTP) [14, 18, 19] and untreated water [17]. Owing to this, the conventional methods of wastewater treatment have been faulted for their inability to completely eradicate Phcs, leading to their release into nearby rivers and coastal areas [20]. Other non-point sources of Phcs within the environment are connected to the inappropriate disposal of expired drugs, human feces, agriculture, and veterinary practices, leaching into surface and ground waters [10, 19, 21]. Some Phcs like paracetamol, ciprofloxacin, sulfamethoxazole, and caffeine are readily biodegradable and can be significantly removed during treatment [21], while some degrade slowly [22]. Furthermore, some could be stable in the environment and varying concentrations have been uncovered within the range of ngL−1 to μgL−1 in the effluents of WWTP, contaminating water bodies around the world [6, 19], [23,24,25].

General Wastewater Treatment Methods

In the past, different conventional methods have been considered for eliminating Phcs from wastewater, they include coagulation [26], advanced oxidative process (AOP) [27], biological method [28], ozonation and filtration [17, 27]. Coagulation is known for treating wastewater with the use of chemicals known as coagulants that bind pollutants until a huge mass that can be separated via settling is formed [29]. Common coagulants used are salts of aluminum and iron, although there are issues associated with the disposal of sludge generated from this method [29]. AOP also utilizes chemicals to remove inorganic or organic pollutants from wastewater, resulting in the formation –OH radicals [29]. AOP’s potential to breakdown complex compounds present it as an efficient technique for removing Phcs from wastewater [30]. However, its enormous operation cost [27], huge energy consumption [31], and the need for a spacious set-up increases its process cost and hinders its use globally [27]. Biological technique utilizes the activity of microbes in the treatment of wastewater, it is an eco-friendly technique, nonetheless, it takes time and there are concerns on how to manage the sludge generated when it is used [29], besides, not all Phcs are removed using this technique [14]. Ozonation is beneficial in that it produces no sludge, nonetheless, it produces other by-products in WWTP effluents [29]. Reportedly, these by-products could be more toxic when compared to the initial Phcs contaminants, however, it remains a viable wastewater treatment option [31]. Filtration utilizes a membrane (Constro [32] attached to leaky support and is useful for taking out dissolved contaminants during the treatment of wastewater [29]. Nonetheless, for membrane-filtration, there could be a need for frequent membrane replacement [29], and the problem of fouling [28]. In themselves, each of the conventional techniques may not be sufficient for fully removing Phcs from wastewater [12, 14, 33], and a summary of their demerits is presented in Fig. 1. Hence, with their shortcomings, it is expedient to complement these methods to improve the percentage of Phcs being removed during treatment [34]. Of recent, the interest in the use of adsorption for improving the removal of Phcs from wastewater [17, 35, 36] is rising [37], and this is because it is efficient [36], environmentally benign [38], and easily operated [39]. Albeit there are concerns about the huge cost of adsorbents like commercial activated carbon, paving way for more research on less-costly ones like biochar [40]. Biochar has been suggested suitable for removing Phcs in an adsorption process [17]. When compared to commercial-activated-carbon, the cost, energy-use, and greenhouse gas (GHG) emissions during the production of biochar is lesser. This is illustrated in Fig. 2.

Fig. 1
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Some demerits of conventional methods of wastewater treatment

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Fig. 2
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Source of information.

Some performance metrics during biochar/activated carbon production. [29, 90,91,92,93,94]

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Estonia and IB, MF& DF Phcs

Estonia is one of the countries within the North-Eastern region of Europe. It lies within the latitude and longitudinal points of 59.5° N 28° E and 57.5° N 22° E respectively. Its vegetation zone is known as hemiboreal with a relatively flat landscape, albeit its south-eastern region is more mountainous [41]. There are four seasons in Estonia, spring starts from March–April, followed by summer from June–August, then Autumn from September–November and cold winter from December to February [41]. According to Statistics, Estonia’s estimated population is 1.32 million [42], and the life expectancy of Estonians has shown a positive trend in the last decade as people are expected to live much longer; between 70–74 and 80–82 years for males and females respectively (Fig. 3). With the extended life expectancy, the consumption of IB and other medications to maintain a healthy lifestyle will increase [43]. Furthermore, according to the data available from Estonia Statistics on medicine, the consumption of different prescribed and over-the-counter drugs has risen significantly [2, 44]. Besides, three major Anatomical Therapeutic Chemical (ATC) groups of Phcs (such as alimentary tract drugs and nervous system pharmaceuticals [“non-steroidal anti-inflammatory drugs” (NSAIDs)] were frequently consumed among Estonians between 2016 and 2018, with MF, DF, and IB falling under this groups of Phcs [45, 46]. Hence, they are now frequently detected in municipal WWTP influent and effluent streams [47], resulting in their prevalence within the environment. Besides, with Estonia being one of the countries in the Baltic Area, data analysis on existing data confirmed that NSAIDs, epilepsy, diabetes and cardiovascular disease medicine were the most used, of which MF, IB and DF belongs to one of these class of drugs [18]. MF is an antidiabetic medicine [22] and is amongst the most detected Phcs in the effluent of WWTP [48]. According to statistics from the international diabetes federation, above 400 million persons worldwide live with diabetes [49], 59 million patients are in Europe and 58,700 cases in Estonia; which equals about 6.2% of the adult population in Estonia [50, 142]. MF’s prescription has increased significantly owing to its efficiency in glucose removal via acting on metabolic paths to encourage catabolism and the elimination of glucose [48]. Furthermore, in Europe (including Estonia), the consumption of MF will likely increase, because over 68 million people would reportedly have this disease by 2045 [51]. A general report on the fate of Phcs across the Baltic States shows that MF’s highest peak concentration exceeded 1 mg/l in WWTP influent while a peak optimum and average concentration of 0.92 mg/l and 0.16 mg/l respectively were detected in their effluent streams [47]. DF is a phenylacetic acid derivative, while IB is generally accessible as 2-(4-isobutylphenyl) propionic acid. Other properties of IB, DF and MF are presented in Table 1. Between 2006 and 2014, there was a significant 14.1% increase in IB consumption in Estonia [43], while the devastating side effects of DF (such as stroke and heart attack in a patient with certain cardiovascular risk factors) has prompted the European Medicine Agency to recommend that DF should be utilized at the lowest active dose [43]. Furthermore, the result of a survey in Europe found the peak absolute concentration of IB (48 μg/L), together with DF (11 μg/L) within the secondary effluent of WWTP [11]. DF has also been included as one of the emerging Phcs into the environment by the European commission, and has resulted in an effort to completely remove DF and other Phcs like MF and IB during the treatment of wastewater [14].

Fig. 3
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Life expectancy graph for Estonia between 2010 and 2019. Source. [95]

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Table 1 Some properties of IB, DF, and MF
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Estonia and WWTP

There are different reports on the number of WWTP in Estonia, however, from the literature that was found, we can conclude that the number varies from 664 to 730 [52,53,54]. Based on the report of [52], the capacity of each of the WWTP varies from 300 to > 100,000 PEs (person equivalents). The breakdown is presented in Table 2. Besides, the main wastewater treatment technologies utilized at Estonia’s WWTP are presented in Fig. 4, where the use of batch and plug flow reactors fall under the activated sludge technology, oxidation ponds and wetlands fall under the natural solutions while others are the biofilm reactor technology. However, these technologies utilize a combination of physical, biological, and chemical treatment methods (including those mentioned in “General wastewater treatment methods” Section) that are deemed insufficient to totally take out Phcs in WWTP. This further buttresses the need to complement these existing methods (in “General wastewater treatment methods” Section) with adsorption using lower-cost biochar, which is cheaper when compared with activated carbon for improving the removal of Phcs.

Table 2 Estonia’s WWTP and their capacities
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Fig. 4
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Treatment technologies at Estonia’s WWTP

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Analysis of Phcs in Wastewater

To ensure the complete removal of Phcs during the treatment of wastewater, they need to be properly analyzed to ascertain their actual concentration within the influent and effluent streams of WWTP. Some of the common techniques for the analysis of Phcs are presented in Table 3.

Table 3 Some common techniques for the analysis of Phcs in wastewater
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Methodology

For database analysis, the Scopus database was used to establish the need for a synopsis on the possibility of using biochar to augment the removal of Phcs in Estonia’s WWTP. Several searches were conducted using different keywords. The first search was steered at establishing the rising interest in the utilization of biochar for adsorption. Using biochar+adsorption as the main search word, bounded using the title of publications, abstracts, and keywords between 2005 and 2021. 5980 research documents were found. When Estonia was added to the main search words (biochar+adsorption+Estonia) under similar search conditions, no document was found. Although there might be a few documents on other databases, however, this result reflects the state of biochar research in Estonia. The second search was carried out using wastewater treatment+biochar as the main words under similar search conditions, and 1523 documents were found. This is lower than the result of the first search, and based on this, we concluded that the research into using biochar in wastewater treatment is just budding. Like the first search, when Estonia was added to the main search words (wastewater treatment+biochar+Estonia), there was no document found. Other searches were conducted to study the extent of research into using biochar for the adsorption of IB, DF, and MF, and the results are shown in Fig. 5. From the results, there was a confirmation of the need to present a summary of the potential of biochar as an adsorbent for Phcs uptake. This was later narrowed down to three of the most prevalent Phcs (IB, DF, and MF) in Estonia.

Fig. 5
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Database analysis graph. B Biochar, A Adsorption, E Estonia, WT Wastewater treatment, W Wastewater, Phcs Pharmaceuticals, T Treatment, D Diclofenac, I Ibuprofen, M Metformin

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Biochar and Production

Biochar is rich in carbon and produced via the pyrolysis of various feedstocks, ranging from virgin to waste biomass. This comprises of agricultural plantings, manures, farm wastes, and municipal wastes [40, 55]. Some of biochar’s remarkable properties as shown in Fig. 6 present it as a suitable material in diverse applications, including being used as an adsorbent in treating wastewater. Furthermore, the possibility of utilizing low-cost and easily accessible biomass wastes as its precursor cements biochar as a sustainable adsorbent that can be used during wastewater treatment [56].

Fig. 6
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Suitable biochar properties as an adsorbent for Phcs uptake

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Biochar is produced via several thermal-chemical processes [57]. It includes torrefaction, and gasification, together with pyrolysis (Fig. 7). The choice of thermochemical process and process parameter is vital during biochar production [40]. Generally, torrefaction is used in converting biomass feedstock into a stable solid that can be used as an energy fuel [58], it could either be a dry or wet process [59]. Dry torrefaction is used to pre-treat biomass [60]. During this process, biomass is warmed-up between 200 and 300 °C, in an inert setting for about 0.5 to a few hours [61]. Although dry torrefaction produces a biochar-like substance (known as torrefied biomass) [62], however, it is best to say that torrefaction is for improving some of the properties of raw biomass before being used in energy applications [59].

Fig. 7
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Common thermochemical techniques for biochar production

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Wet torrefaction, on the other hand, is also known as hydrothermal carbonization [61]. It is utilized for charring wet biomass like forestry residue and animal manure [63]. The temperature at which hydrothermal carbonization takes place is relatively low (typically between 180 and 265 °C) [59]. Furthermore, its feedstock does not require drying, thus, reducing energy consumption and production cost [61]. The process takes place within the saturation vapor pressure of water (2–10 mega-pascal) [61] and at time intervals of 5 min [59] to hours [64]. The products of hydrothermal carbonization include hydrochar (slurry-char), bio-oil (liquid), and a small amount of gas (majorly CO2) [62], [61]. Nonetheless, the problems of reactor corrosion, deposition, clogging, and high-pressure requirements limit the utilization of wet torrefaction [61].

Gasification is another thermochemical process [65]. Unlike torrefaction, gasification requires partial oxygen [65], and an elevated temperature of 500–1400 °C [66] within a short residence time, typically between 10 and 20 s [67]. The main target of biomass gasification is to produce gas mixtures of methane (CH4), alongside carbon monoxide (CO), and hydrogen (H2), and a small amount of carbon dioxide (CO2) [66]. Although it produces biochar as a byproduct at a scanty rate (typically at a yield of about 5–10%) [68]. Similarly, biochar made from biomass gasification contains various inorganic elements, together with polycyclic-aromatic-hydrocarbons (PAHs) [67, 69]. The PAHs produced are harmful, hence limiting the utilization of gasification’s biochar for environmental remediation purposes [70].

Pyrolyzing biomass is mostly known as the breakdown of biomass raw material within a zero oxygen setting using heat [68]. It is the main technique for producing biochar, such that biomass disintegrates into vapor [condensable (bio-oil, tar) and non-condensable (syngas)], and solid (biochar) [71]. The temperature interval during pyrolysis usually lies between 300 and 1000 °C [71]. It is cost-effective and possesses the ability to produce a variety of products that can be used in different applications (syngas and bio-oil can be converted into energy fuels) [72, 73]. Based on the conditions of operation, pyrolysis can be mainly as fast and slow type [71]. Fast pyrolysis takes place at a temperature interval of 400–600 °C, high heating rate (typically greater than 300 °C/min), and little vapor-residence time (typically between 0.5 and 10 s). The main target of fast pyrolysis is bio-oil, although biochar is formed as a by-product (about 15–30 wt.% yield) [67, 71]. Conversely, the slow form of pyrolysis is the choicest for producing biochar. This is because it produces the highest amount of solid product (typically between 35 and 50 wt.%) [67, 71]. It occurs between 300 and 800 °C [71], with a heating rate of 5–10 °C/min, and a few minutes to hours of residence time [67, 74].

Biochar Characterization

In understanding the features and possible application of biochar, it is subjected to various characterizations. Some of the characterization techniques include elemental, proximate, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), X-ray Diffraction (XRD), X-ray photoelectron spectrometer (XPS), Fourier transform infrared spectroscopy (FTIR), pH, and energy dispersive X-ray (EDX) analysis [75]. Elemental analysis is utilized in determining the carbon, hydrogen, nitrogen, oxygen (by difference) [76], and sometimes sulfur content of biochar samples [77]. Proximate analysis is influential in determining the volatile matter, moisture, and fixed carbon, together with ash [76]. TGA assists in understanding the thermal decomposition [78], stability [79], and kinetic pattern during the pyrolysis of biomass to form biochar [80]. SEM is an analysis that is useful in detecting biochar’s superficial morphology and structure [79]. BET is a valuable technique for determining the pore size together with the pore volume, and the surface area of biochar [79], XPS is functional in evaluating the elemental constituent (albeit at the surface alone) [79], while XRD is useful in determining the crystalline and amorphous phase present in biochar [81]. pH analysis detects the extent of acidity or alkalinity of a biochar material, and FTIR is beneficial in determining the functional groups that are on biochar’s surface [75]. EDX is another method for checking biochar’s elemental constituent [75], ThermoFisher [82]. Other characterization techniques include “inductively coupled plasma mass spectrometry (ICP-MS)” [83], electrical conductivity [84], and Raman spectroscopy analysis [85]. It is essential to note that the choice of characterization depends on the projected application and utilization of biochar. For using biochar as an adsorbent in WWTP, some of the reported analyses are mentioned in Table 4.

Table 4 Reports on biochar production, characterization, and utilization for Phcs adsorption
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Phcs Adsorption Using Biochar

Globally, carbon-containing materials are used in the adsorption process, with activated carbon (AC) being the most prevalent [40]. However, the cost of its production is high and has resulted in the search for a low-cost alternative [30, 86]. Recently, biochar that possess comparable adsorbent properties to AC has been considered an economically viable option [30, 86]. The slow pyrolysis of biomass and biomass wastes under restricted oxygen condition produces biochar that has recently been described as an appropriate and low-cost adsorbent for improving Phcs removal from WWTP using adsorption [55, 87]. Furthermore, Oliveira et al., elucidated that biochar is a cheap substitute for activated carbon for removing diverse environmental contaminants like Phcs [88]. Some existing reports on the use of biochar for Phcs uptake are mentioned in Table 4 Further reports on the use of biochar for the uptake of MF, DF, and IB are presented in Tables 5, 6, and 7 respectively.

Table 5 Biochar for MF uptake
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Table 6 Biochar for DF uptake
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Table 7 Biochar for IB Uptake
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Adsorption is a surface phenomenon such that the adsorbate lies on the adsorbent either through a chemical or physical mechanism [89]. The possibilities of utilizing biochar for Phcs adsorption have been attested to at laboratory scale, with reports of several mechanisms that govern their adsorption onto biochar’s surface. Some of these mechanisms are highlighted in Tables 4, 5, 6, and 7. From these, a summary of the prevalent mechanisms controlling the adsorption of Phcs including MF, DF, and IB onto biochar’s surface is presented in Fig. 8.

Fig. 8
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Prevalent mechanisms governing Phcs (including MF, DF, and IB) adsorption onto biochar

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Recommendations

From the summaries provided, we can conclude that it is possible to utilize biochar for improving the removal of Phcs, including MF, DF, and IB during the treatment of wastewater at WWTP. Hence, there would need to be more publicity on this possibility. Furthermore, there is a need for proper education on Phcs disposal within the environment, alongside the danger it poses if it is improperly disposed of. One of the limitations of this study is the concern on the recovery and re-utilization of spent biochar during wastewater treatment, however, there are a few reports on regenerating spent biochar during adsorption as reported in Table 8. Another concern is on augmenting biochar’s physiochemical features to ensure that they perform extremely well as an adsorbent. Most of the studies reviewed had to improve biochar’s features for better adsorption performance via activation using physical or chemical means. Hence, this could result in a rise in the overall cost of producing and utilizing biochar for Phcs adsorption. Nonetheless, a cost–benefit analysis should suffice in establishing the trade-off between the cost of biochar production and activation during Phcs’ adsorption. For future study, a feasibility study should suffice to ascertain the cost-effectiveness of using biochar for improving the removal of Phcs at WWTP. To do this, a process model that reflects the operations of a WWTP would be very useful. Lastly, most biochar-adsorption experiments have been studied at laboratory scales and there should be a further step in trying it out at pilot and industrial scales. Lastly, asides the possibility of using biochar in an adsorption process, its production from biomass wastes pave a path for a bio-circular economy universally.

Table 8 Some biochar regeneration techniques
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Conclusions

The danger of Phcs within the environment has been expressed in this synopsis, with emphasis on MF, DF, and IB in Estonia. The effluents from WWTP have been identified as one of the point carriers of Phcs into the environment, hence, the need to improve existing treatment methods for removing Phcs during wastewater treatment at WWTP. In contrast to the idea of replacing the existing methods of wastewater treatment, it is better to complement them with adsorption. This is because adsorption alone would not be sufficient for improving Phcs removal at WWTP. Owing to this, the possibility of using adsorption with biochar being the adsorbent for improving pharmaceutical removal has been stated in this work.