The possible use of hydrochar and biochar for wastewater treatment is justified by the low production costs and by the intrinsic properties of this material. The high specific surface (Sbet), the porosity, and the presence of functional groups that can bind many molecules are important properties of effective adsorbents (Titirici et al., 2007). The use of microalgae as feedstocks for HTC in this field was recently proposed by Sun et al. (2018). This study concerned the production of a porous carbon-made adsorbent using Chlorococcum spp. to remove Cr (VI) from water solution. A mixture made of 2.25 g of dried microalgae and 0.7 g of oxalic acid was resuspended in 0.3 L of deionized water and heated at 200°C for three hours. The obtained hydrochar was collected by filtration and dried at 110°C. Then the sample was post-treated via KOH or NH3 activation, following heating ramps at 650°C, 700°C, 750°C, or 900°C under an N2 stream (120 mL min− 1). Undoubtedly the activation step is crucial in this study to achieve a higher specific surface area and a higher sorbent capacity. The activation via KOH at 750°C allowed a Sbet increase of two orders of magnitude (from 11 m2 g− 1 to 1784 m2 g− 1). The adsorption isotherms on Cr (VI) were determined for all the obtained hydrochars and different results were obtained among the tested samples. The KOH-activated hydrochar showed the best adsorption capacity (Qe) (370.37 m2 g− 1). The NH3-activated hydrochar and the nonactivated hydrochar had Qe = 95.60 mg g− 1 and 17.60 mg g− 1, respectively.
The effectiveness of the adsorbent was increased through post-treatment, however, energy consumption, use of solvents, cleaning steps, and N2 consumption also increase the overall cost in the view of a scale-up. This is also a limit for similar applications. That’s why the scientific community is now focusing on alternatives to produce new low-cost sorbents with suitable characteristics for the adsorption of contaminants from water.
Similar research was carried out by Saber et al. 2018, working with microalgal-based hydrochar obtained by hydrothermal liquefaction to remove Cu (II) ions from an aqueous solution. 10 g of dried Nannochloropsis spp. were used and resuspended in 150 mL of deionized water. Then, the obtained suspension was heated in an autoclave at 210 and 250°C for 60 minutes. Argon was used to create anoxic conditions in the reactor vessel. The used thermal treatment should have favored the formation of the liquid byproduct at the expense of the hydrochar, however, the temperature conditions are still considered by some authors as typical of hydrothermal carbonization. The obtained hydrochars had a BET surface area of 1.38 and 12.56 m2 g− 1, respectively. Both samples were tested for the removal of copper, using a 5g L− 1 dose, studying the effect of pH, initial Cu (II) concentration, and contact time. Experiments were performed in the pH range 2–6 with a Cu (II) starting concentration of 100 mg L− 1. The maximum adsorption capacity was obtained at pH 5 and the best performances were provided by the hydrochar produced at 250°C (19.7 mg g− 1 against the 14.7 mg g− 1 of the other one). Furthermore, by increasing the starting copper concentration (up to 300 mg L− 1) higher adsorption was achieved with a maximum value of 35 mg g− 1 after 24h.
The already cited work of Aron et al. (2020) focused on nutrient recovery from wastewater using microalgal-based hydrochar as an adsorbent prepared at 350°C. The factors affecting adsorption were studied, testing different contact times (10–60 min), speed (50–250 rpm), temperature (10–60°C), and adsorbent dose (0.3–1.5 g L− 1). The adsorption tests were performed on a synthetic solution having a COD, phosphorus, and ammoniacal nitrogen concentration of 5000, 150, and 80 mg L− 1, respectively. Phosphorus and ammoniacal nitrogen removal were high (90 and 73% respectively) at 60 min at 1g L− 1 dose.
Yu et al. (2021) proposed a similar concept by preparing a microalgal-based hydrochar through wet torrefaction, a synonym for hydrothermal carbonization, for the removal of Methylene blue (MB) and Congo red (CR) dyes from water solutions. The hydrochar was prepared by treating Chlorella spp. in a modified household microwave by creating an inert atmosphere through nitrogen purging. A final temperature in the range between 160–170°C was maintained for 5 minutes, allowing the production of hydrochar with an SBet of 2.66 m2g− 1 and an average pore size of 0.65 nm in diameter. Laboratory batch experiments were performed to remove the two dyes from water solutions, by testing different doses, pH, contact time, and the performance of the hydrochar on different pollutant concentrations. Removal efficiencies increased by using higher hydrochar doses. Maximum efficiencies of 94 and 98% were obtained for MB and CR using a 5 gL− 1 and a 3 gL− 1 concentration of hydrochar respectively. The optimum pH for removal of MB was 2–8, whereas for removal of CR was 6–8. The adsorption experiments tested a range of concentrations from 5 to 550 ppm.
The same hydrothermal process used by the previous authors was adapted to prepare a magnetic adsorbent material using blue-green microalgae as a carbonaceous substrate under different loadings of iron (Peng et al., 2014). The aim was to obtain a porous material to remove Tetracycline (TC) from aqueous systems. Tetracycline is a common antibiotic that shows bacteriostatic activity against a wide range of gram-positive and gram-negative bacteria. 3 g of dried microalgae were mixed with different amounts of an iron salt (0.6, 1.2, 2.4, 3.5 g of (NH4)2SO4·FeSO4·6H2O) and homogenized in 0.1 L of distilled water. The obtained mixture was put in a stainless-steel reactor and heated in an electric oven for 6 h, setting a temperature of 170°C. Once the reactor had cooled, the mixture products were centrifuged to recover the solid fraction, then washed with distilled water and ethanol (99%) solution, and finally dried in a vacuum oven at 60°C for 24h. The SEM analysis showed the presence of Fe3O4 nanoparticles dispersed in the carbonaceous substrate. The starting iron concentration used for the synthesis emerged as a leading factor in the morphology and properties of the samples. Not only the presence of iron make easier the magnetic separation and recovery of the particles after the treatment but also hydrochar conformation changed from sheets to particles with the increased iron load. The authors refer to that tendency as the “separation effect”: the iron ions in the reactor vessels modify the carbon structure aggregation, creating more defective sites and structural complexity in the matrix. This was confirmed also by surface area data (Table 4) which showed a positive correlation with the starting iron concentration.
Table 4
– Specific Surface area and pore volume of the samples as a function of the added iron dose (Peng et al (2014), modified)
(NH4)2SO4·FeSO4·6H2O)
(g)
|
mg Fe/g on
dried microalgae
|
Surface Area
(g m− 2)
|
Pore Volume
(cm3g− 1)
|
0.6
|
28
|
38.3
|
0.012
|
1.2
|
56
|
96.5
|
0.026
|
2.4
|
112
|
122.8
|
0.027
|
3.5
|
168
|
128.3
|
0.029
|
The different iron-doped biochar prototypes were tested at the dose of 2 g L− 1 for the removal of TC, running some jar tests on a 50 mg L− 1 water solution of TC. The samples were analyzed with UV–vis spectroscopy at 360 nm wavelength. The greater TC removal efficiency (97.8%) was obtained using the sample made with 3.5 g of (NH4)2SO4·FeSO4·6H2O (6 mmol).
During the hydrothermal carbonization, the iron salt interactions with the carbonaceous matrix have resulted in the formation of iron oxide nanoparticles. This was certainly a great advantage with respect to the work of Sun et al (2018). Positive correlations were highlighted between the starting iron amount and the specific surface area as well as between the starting iron amount and the sorption capacity of the final product. The results suggest that iron leads to valuable hydrogen bonding and other chemical-physical interactions with the TC.
Mantovani et al., 2022 suggested valorizing the microalgal biomass grown on municipal wastewater at a pilot scale to produce microalgal-based iron nanoparticles (ME-nFe). HTC was carried out at 225°C mixing the microalgae with iron nitrate. The final solid product, having a surface area of 110 g m− 2, was used as an adsorbent at a laboratory scale. Jar tests were carried out with a ME-nFe dose of 3 g L− 1 to remove copper, zinc, cadmium, nickel and total chromium (at a starting concentration of 1 mg L− 1) from water solutions and treated effluents of a municipal wastewater treatment plant in Milan (Italy). The removal of copper, zinc and cadmium from the treated effluent was over 96%. Nickel removal was slightly lower (80%) but negligible for total chromium (12%). By exploiting the magnetic properties of the zero-valent iron in the ME-nFe, the same batch of adsorbent could be recovered and used effectively for three consecutive Jar tests.
The use of iron for wastewater purposes is historically well known (Fu et al., 2014), but more recently iron oxide and zerovalent iron nanoparticles have created a great deal of interest in the wastewater treatment field because of their high potential. Zero-Valent Iron (ZVI) is a quite abundant chemical element, it is not toxic, cheap, and easy to produce. In the past, it was tested for wastewater treatment applications. ZVI was used to transform many pollutants into not toxic or less toxic compounds, exploiting reductive processes, opposite to the most common oxidative one used in wastewater treatment. ZVI has proven effective in removing a wide range of chemical contaminants from aqueous solutions (chlorinated organic compounds, metals, and pharmaceutical compounds). However, the rise of nanotechnologies shifted the focus of the scientific community as nanomaterials often show very different and more interesting physical, chemical, and biological properties compared to their macroscopic counterpart. This results from their larger surface area per unit of volume and the quantum effects (Khan et al., 2019). Promising nanoscale solutions, which can be applied to water treatment, already exist. However, the current cost of these materials commonly prevents their use for large-scale applications. Thus, developing low-cost nanotechnology to remove pollutants from water is still an important goal.
Within this context, zerovalent iron nanoparticles (nZVI) have been tested in the last years as an interesting technology to remove several classes of contaminants from wastewaters. Since they are characterized by a high reductive potential, they can offer a new perspective to water treatment, traditionally focused on oxidative processes. What makes nZVI so promising in this field of application are the high specific surface area and their enhanced reactivity which allow them to act both as an adsorbent and as a reducing agent. Indeed, literature data testify to good results in the removal of chlorinated compounds and heavy metals in aqueous solutions (Kharisov et al., 2012).
In the last twenty years, a lot of reports on different ways to synthesize iron nanoparticles have been described, following two different general approaches: the top-down synthesis involves a starting macro or microscopic substrate which is broken down using physical or chemical methods to obtain nanostructures. The bottom-up synthesis produces nanomaterials by building-up atoms or molecules onto each other. The most used methods are thermal reduction and/or decomposition of iron salts with sodium borohydride, carbothermal synthesis, vapor-solid growth synthesis, and sol-gel techniques (Ahmmad et al., 2013; Crane and Scott, 2012).
The chemical reduction of iron salts in water solution is probably the most common method for the synthesis of iron nanoparticles. Sodium borohydride (NaBH4) is used as the reducing agent according to the following reactions:
$${Fe({H}_{2}O{)}_{6}}^{3+ }+3B{H}_{4}^{-}+3{H}_{2}O\to {Fe}_{\left(s\right)}^{0}\downarrow +3B(OH{)}_{3}+10.5{H}_{2\left(g\right)}+3{H}^{+}$$
1
$${Fe({H}_{2}O{)}_{6}}^{2+ }+2B{H}_{4}^{-}\to {Fe}_{\left(s\right)}^{0}\downarrow +2B(OH{)}_{3}+7{H}_{2\left(g\right)}$$
2
This technique produces very small particles with a homogeneous structure, showing high reactivity and high specific surface area. The drawbacks include the use of sodium borohydride and other organic solvents, which increase the costs of the process making it difficult to scale up to an industrial level (Li et al., 2009).
The carbothermal synthesis of nanoparticles consists of the reduction of metal oxides using a gaseous stream of H2 or CO2 to achieve reducing conditions in the reaction vessel. The gas could also be CO produced directly in the reactor through the thermal decomposition of a carbonaceous substrate under high temperature (> 500°C) (Stefaniuk et al., 2016).
The vapor-solid growth synthesis consists of the deposition of gas onto solid support allowing the formation of crystals and the condensation of metal nanoparticles (Pandey et al., 2011). The Sol-gel method involves three subsequent steps: a solution containing the precursor metal is first hydrolyzed to obtain a hydroxide solution which is converted into a gel phase through condensation. Finally, the drying process favors the formation of ultrafine powders (Qi et al., 2010).
However, these methods are quite expensive and usually require advanced equipment to reach high temperatures and pressure. Furthermore, a separation procedure is usually needed to recover the final product from the reaction medium as well as a post-treatment to remove impurities deriving from the use of solvents and chemical additives.
Moreover, the application of nZVI for wastewater treatment also involves other problems. Their rapid oxidation, the difficulty to separate nZVI from the treated media as they tend to aggregate in water solution and the “aging effect” are serious issues that limit their efficiency (Calderon and Fullana, 2015). Considering the agglomeration tendency of nZVI in water solution, a recent but promising strategy consists of the combination of the nanoparticles with porous material acting as physical support.
In the last years, the scientific community focused on new strategies for the synthesis of iron nanoparticles, aiming at reducing costs while overcoming the above-described limitations. An important turning point is certainly the possibility of using compounds deriving from renewable sources. Biomasses are rich in antioxidants which can be exploited as reducing agents, modifying the conventional synthesis of nanoparticles. The entire production can take a new eco-friendly perspective by replacing chemical additives like NaBH4 with the antioxidants naturally found in the biomass. The study of Peng et al. (2014), previously described, fits perfectly that framework.
Furthermore, to improve the effectiveness of nZVI, also modification of iron nanoparticles has been investigated, improving their chemical-physical characteristics and their potential for wastewater treatment purposes. Various publications deal with different strategies involving the addition of alumina (Karabelli et al., 2011), polymers (Jiang et al., 2011,) and carbonaceous materials (Vadahanambi et al., 2013; Wildgoose et al., 2006). Many waste biomasses are quite rich in carbon and can be used as such but can also be pretreated and used to produce modified nanoparticles.
A different concept was proposed by Lalhmunsiama et al. (2017), who produced a sorbent magnetic material combining some already synthesized iron oxides nanoparticles (Fe3O4) with microalgae. The process was carried out in two different steps: the iron nanoparticles were initially produced with the coprecipitation methods consisting of the dispersion of iron oxides (Fe2+ and Fe3+) in an ammonium hydroxide water solution. The suspension was stirred and heated at 80°C until the formation and precipitation of the nanoparticles were achieved. Then, 1 g of Fe3O4 nanoparticles was dispersed in 0.2 L of distilled water with 5 g of dried microalgae (Chlorella vulgaris). The mixture was stirred for 24h to allow the deposition and incorporation of the iron nanoparticles on the biomass surface. As a result of that protocol, it was possible to limit the agglomeration of the nanoparticles in water as well as decrease of the active sites. The effect of various physico-chemical parameters like pH, initial concentration, contact time, and background electrolyte concentrations was studied under the batch reactor studies. The obtained material was found to be efficient in the rapid uptake of Cd (II) and Pb (II) from aqueous solutions. Table 5 summarizes the main literature findings concerning the use of microalgal-based hydrochar as an adsorbent.
Table 5 – Use of microalgal hydrochar as adsorbent, as such and doped, for the removal of pollutants from water.
Microalgal species
|
Production process
|
Application rate
|
Pollutant removal
|
Reference
|
Chlorococcum spp.
|
HTC, T=200 °C, 3h residence time,
Post-treatment with KOH at 750 °C under N2 stream
|
0.2 g L-1
|
Cr (VI)
|
Sun et al.,2018
|
Nannochloropsis spp.
|
HTC/HTL, T=200, and 250°C, 1h residence time
|
5 g L-1
|
Cr (VI)
|
Saber et al., 2018
|
Scenedesmus spp.
|
HTC/HTL, T=350°C, 1h residence time
|
1 g L-1
|
Phosphorus
And Ammoniacal Nitrogen
|
Aron et al., 2020
|
Chlorella spp.
|
HTC, T=160-170°C, 5 min residence time in a modified microwave
|
5 g L-1
5 g L-1
|
Methylene Blue
Congo Red
|
Yu et al., 2021
|
Blue-green microalgae
|
HTC, T =170 °C, 6 h residence time
|
2 g L-1
|
Tetracycline
|
Peng et al., 2014
|
Chlorella spp. and Scenedesmus spp.
|
HTC, T =225 °C, 3 h residence time
|
3 g L-1
|
Cd(II), Ni(II), Cu(II), Zn(II) and Total Cr
|
Mantovani et al., 2022
|
Chlorella vulgaris
|
Co-precipitation method at 80°C + deposition on algal biomass
|
2 g L-1
|
Cd (II) and Pb (II)
|
Lalhmunsiama et al., 2017
|