Adsorption efficiency of biochar produced by aquaculture by-products for removing geosmin in aquaculture environment

The conversion of biomass energy into biochar through carbonization is now a globally recognized process that can mitigate the problem of climate change, and this process has a clear trend of easy access to raw materials and safe and clean production (Lehmann & Joseph 2015). This can be promoted and used in the fields of ecology and environment, which can sequester carbon and reduce emissions, and can be integrated with agriculture and forestry to alleviate a series of problems caused by waste and greenhouse gas pollution (Liang et al. 2006). Using biochar on the field can improve the soil, increase crop yield, and improve the process of sustainable agricultural development, and it also plays a great role in heavy metal pollution adsorption and water purification (Steinbeiss et al. 2009). The comprehensive utilization of biochar can be conducive to energy saving and consumption reduction, and it promotes environmental protection and governance, which is of great practical significance to the national environment, energy, and food security (Duan et al. 2007). In recent years, China's aquaculture industry has seen rapid development and large-scale expansion; however, pond aquaculture in guaranteeing the effective supply of aquatic products at the same time, the weak infrastructure, and the general lack of drain water treatment facilities of the short board have also been gradually exposed. The ecological environment of the watershed has serious negative impacts, such as aquaculture by-products (moss, Elodea, etc.) that cannot be effectively dealt with, wastage of a large number of resources, the process of aquaculture odor produced by the environmental materials and human beings have caused a certain degree of harm but have not been properly dealt with, and the breeding of wastewater pollution of the environment (Nie et al. 2023). To better recycle the drain water and achieve resource recycling, aquaculture water treatment should take into account a variety of influencing factors, to minimize the introduction of additional substances that cause pollution or affect the existing ecological environment as far as possible. This is the first study to utilize aquaculture by-products in the production of biochars and then to adsorb odorant substances from drain water, aiming to achieve efficient purification of odorous substances in aquaculture drain water while realizing the recycling of resources and helping the green and high-quality development of fisheries.

Odorants have become one of the water pollutants that have been an increasing concern in recent years (Abd El-Hack et al. 2022). In aquaculture, these odorous substances can enter the bodies of aquatic products, which seriously affect the development of aquaculture as well as bringing serious problems to the sale of aquatic products (Wilkes et al. 2000). These off-flavors often lead to a loss of consumer interest in consuming fish and shellfish (Sarker et al. 2014). Geosmin (GSM) belongs to the monoterpenes of terpenoids, and this terpene alcohol is usually a secondary metabolite of fungi, slime molds, actinomycetes, and cyanobacteria (Gerber & Lechevalier 1965; Boerjesson et al. 1993). In our country, GSM produced by algal metabolism are important causes of olfactory problems in drinking water. So far, more than 200 kinds of algae have been found to produce GSM, including some algae of the genera Schizophyllum, Tremella, Sphingomonas, and fishy algae, and GSM is mainly produced by fishy algae (Jüttner & Watson 2007). Seasonal concentrations of GSM correlate well with the abundance of Aphanizomenon (Jüttner et al. 1986) and Anabaena (Jones & Korth 1995) in nutrient freshwater lakes. Biochar is a porous carbon-containing material produced during the thermochemical decomposition of biomass feedstocks in the presence of little or no oxygen. Biomass feedstock can be any organic waste material, including crop and forest residues, wood chips, algae, sewage sludge, manure, and organic municipal solid waste (Xiang et al. 2020). Biochar has a relatively large surface area and an abundance of surface functional groups, which allows it to be used as a green and low-cost adsorbent, providing a sustainable alternative to wastewater treatment technologies (Foong et al. 2022). Lin et al. (2021) prepared biochar from pyrolyzed bacterial chaff at 400 °C and studied its adsorption effect on the sediment and water quality of aquaculture ponds, and the results showed that this biochar has great potential as an adsorbent for the adsorption of ammonia, nitrogen, sulfide, and other toxic substances in water bodies and sediments. Liao et al. (2021) showed that the large specific surface area of biochar can enhance nitrous oxide (N₂O) activity and reduce microorganisms, thus mitigating N₂O emissions.

In the pond culture of Chinese mitten crabs and prawns, the main submerged plants are Illicium, black algae, etc. These plants are introduced to improve water quality and provide shelter and food for aquatic animals, but after the culture period, the treatment of the plants is a more troublesome problem. These plants play a key role in water clarity by directly reducing phytoplankton growth through competing with phytoplankton for nutrients and light as well as chemosensory effects (Mulderij 2006; Wang et al. 2006; Kong et al. 2012). They provide a suitable habitat for crabs, especially during molting; crabs may hide in submerged vegetation to avoid predators or other crabs (Wang et al. 2006). They play a key role in the functioning of freshwater ecosystems by influencing the physical and chemical conditions of water and sediments, modifying nutrient cycling, influencing predator–prey interactions, and acting as a source of food, either directly or indirectly through epiphytes (Jeppesen et al. 2012). Aquatic plants have a stabilizing effect on the pond culture system, but too many aquatic plants can lead to eutrophication of the water body and can deprive aquatic animals of their living space (Zehnsdorf et al. 2015).

Moss is a general term for filamentous algae, and some domestic literature points out that it includes water milfoil, bristle algae, double star algae, and transplants; however, its specific species composition and percentage in some specific environments are rarely reported. According to the data, the outbreak of moss in the Great Lakes of North America is composed almost exclusively of a species of aggregate Chadophorasle (Higgins et al. 2008). Bristlenose algae blooms can clog waterworks, produce unpleasant odors, affect landscapes, reduce the ecological value of waters, and pose certain hazards to the aquaculture industry as well (Herbst 1969). Floating algae form loose mats that accumulate on the water's surface. Decay and decomposition of the large number of mats covering the water's surface can cause hypoxia in the water body, which may disrupt natural fluctuations in aquatic animal biomass and elevate the mortality rate of aquatic animals (Norkko & Bonsdorff 1996). In China, moss is a major nuisance in aquaculture, especially in river crab and crayfish breeding ponds. Moss in the pond in large quantities results in thin water, entangled fish and shrimp, a large amount of oxygen consumption at night, the secretion or death of decomposition to produce toxic and harmful substances, and so on. Therefore, moss prevention and control in aquaculture are of great significance.

In pond culture, large amounts of baiting and deposition of fish metabolites lead to the continuous accumulation of organic matter in ponds, resulting in nutrient enrichment of the pond sediment (Dai et al. 2022). This shows that the sediment contains a large amount of biomass.

Moss, excess aquatic plants (mainly Elodea), and nutrient-enriched sediment are all by-products of the aquaculture ecosystem, as are organic waste materials, as mentioned above. These organic waste materials are calcined into biochar under high temperatures and anaerobic conditions, which is used to adsorb odorous substances in aquaculture. The biochar will not cause secondary pollution to the environment. The problem of the aquaculture environment is solved by utilizing the by-products of aquaculture. In this study, moss, Elodea, and pond sediment were made into biochar, respectively, and adsorbed to GSM under the same conditions to investigate their adsorption performance, which provided a solution idea for the by-products in aquaculture to solve the aquaculture environment problem.

In the aquaculture system in general, there are several kinds of biomass raw materials: sediment, algae, moss, aquatic plants, etc. The aquaculture by-products were selected as the raw material for this experiment for the development of biochar to remove the odor from the aquaculture water. The aquaculture of Chinese mitten crabs involves the most biomass and is more representative of the whole aquaculture in China.

The moss, Elodea, and the sediment were dried and placed in an oven at 105 °C for 24 h, and after drying, they were ground into powder in a grinder and filtered in a 100-mesh sieve, which were bottled and placed in a desiccator for spare parts, respectively.

Moss biochar was prepared by high-temperature pyrolysis: moss powder was placed in a 100 mL quartz boat, stuffed as much as possible, compacted, and spread out. The powder was placed in a tube furnace protected by nitrogen (350 mL·min⁻¹) and heated up to the target temperatures (500, 700, and 800 °C) at 10 °C·min⁻¹ for 2 h. It was cooled down naturally to room temperature and then taken out of the tube furnace, put into bottles, and then put into a desiccator for spare use. The produced moss biochar was labeled BC500, BC700, and BC800, respectively.

Elodea and sediment biochar were prepared using the same method with a target temperature of 800 °C and labeled WBC800 and SBC800, respectively.

The field emission electron scanning microscope (SEM) was used to observe the apparent morphological characteristics of the materials and analyze the elemental content by an organic element analyzer; Fourier infrared spectroscopy (FT-IR) was used to analyze the surface functional groups of the materials, and the fully automatic specific surface and porosity analyzer (BET) was used to analyze the specific surface area and pore structure of the materials.

Treatment of the solution to be tested: the use of deodorant standards to prepare a solution to simulate the loose water of the breeding pond containing odorous substances; the use of NaOH and HCl to adjust the pH of the solution; the measurement of 200 mL of deionized water in a 250 mL conical flask; the addition of 600 μL of GSM solution; and then the addition of biochar prepared from different materials to carry out the reaction at 25 °C with 240 r·min⁻¹ in a water bath shaker. Adsorption: After completion of adsorption, the supernatant was aspirated and filtered through a 0.22-μm filter head, and 10 mL was aspirated and kept aside.

Gas chromatography-mass spectrometry (GC-MS) was utilized as the detection method. Also, 10 mL of the water sample was tested by adding 2 g of sodium chloride to a 15 mL headspace injection bottle, screwing the cap tightly, and installing a solid-phase microextraction fiber. The stirring speed was set at 1,200 r·min⁻¹, the temperature was controlled at 60°C, and the extraction time was 40 min. Immediately after the end of the extraction, the extracted fiber was inserted into the inlet of the GC-MS for determination.

The pH of the GSM solution with a concentration of 300 ng·L⁻¹ was adjusted to 7. BC500, BC700, BC800, WBC800, and SBC800 were added to the configured GSM solution at a dosage of 1.0 g, and the remaining GSM concentration was determined by oscillation at a constant temperature of 25 °C for 2 min.

The pH of the GSM solution with a concentration of 300 ng·L⁻¹ was adjusted to 7. BC800 was added to the configured GSM solution at dosages of 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 g, respectively, and the residual GSM concentration was determined by oscillation at a constant temperature of 25 °C for 45 min.

The pH of the GSM solution with a concentration of 300 ng·L⁻¹ was adjusted to 2, 5, 7, 9, and 11, respectively, and BC800 was added to the configured GSM solution at a dosage of 1.0 g. The concentration of the remaining GSM was determined by oscillation at a constant temperature of 25°C for 2 min.

The pH of the GSM solution with a concentration of 300 ng·L⁻¹ was adjusted to 7. BC800 was added to the configured GSM solution at a dosage of 1.0 g. The solution was shaken at a constant temperature of 25 °C for 10 s, 30 s, 1 min, 2 min, 5 min, 7 min, 9 min, 15 min, 30 min, 45 min, and 60 min, and the remaining GSM concentration was determined.

The pH of the GSM solution with concentrations of 100, 200, 300, 400, and 500·ng L⁻¹ was adjusted to 7, respectively, and BC800 was added to the configured GSM solution at a dosage of 1.0 g. The concentration of the remaining GSM was determined by oscillation at a constant temperature of 25°C for 2 min.

Quasi-primary (Equation (1)) and quasi-secondary (Equation (2)) adsorption models were used to fit the kinetic data and analyze the adsorption mechanism of BC800 on GSM.

The Langmuir (Equation (3)) and Freundlich (Equation (4)) isothermal adsorption models were utilized to fit the isothermal adsorption process.

The data analysis in this study was mainly done using Excel 2016 software, and the plotting and equation fitting were done using Origin 2021 software.

The content of each element in biochar can affect its structure and adsorption sites. The pattern of change of the elements in moss biochar varies with the increase of pyrolysis temperature, in which H and O gradually decrease, which originates from the decomposition of organic matter in the process of pyrolysis and the formation of volatile products. The analysis of the elemental contents of the three types of biochars showed that the two types of plant-derived biochars had small differences in the elemental contents of C, H, and N, but differed significantly from that of the sediment biochars. n(O)/n(C) atomic ratio can be used as an indicator to evaluate the degree of aromatization as well as the polarity of the biochars (Zhao et al. 2016). Because the bottom mud biochar had the lowest carbon content, its n(O)/n(C) ratio was the largest at 1.2, followed by BC500 at 0.61, and the smallest at BC800. GSM is a hydrophobic organic pollutant (Liu et al. 2021); when n(O)/n(C) was low, the hydrophilicity and polarity of biochar were also low, and the affinity of water molecules was reduced, which led to the enhancement of adsorption of hydrophobic pollutants. The little change in the C content and the gradual increase in the S content are due to the anaerobic charring condition, which cannot cause C and S to form gaseous oxides and volatilize but to form solid biochar and sulfide, which increase its content due to the concentration effect caused by the decrease of the total amount of content. The C and O contents of WBC800 are much higher than those of SBC800, and the H contents of both are 0. The N and S contents do not change much, indicating that the biomass content of WBC800 is very low (Table 1).

Specific surface area, average pore size, and pore volume of biochar were analyzed using BET. The specific surface areas of BC500, BC700, BC800, WBC800, and SBC800 were determined to be 15.151, 173.831, 205.944, 122.112, and 4.534 m²·g⁻¹, respectively, with average pore diameters of 8.80, 3.52, 3.87, 4.21m and 2.70 nm and pore volumes of 3.33, 1.53, 1.99, 1.29, and 3.06 cm³·g⁻¹, respectively. It was shown that the pyrolysis process results in loss of mass or disproportionate volume reduction of the biomass to form minerals and carbon skeleton but retains the basic pore and structural characteristics of the raw material, so the structure of the biomass feedstock plays a decisive role in influencing the physicochemical properties of the biochar. The smallest specific surface area was found in SBC800, which could be because it had the lowest ash content, which has a large amount of organic matter. The decomposition of organic matter at high temperatures not only generates smoke but also produces a lot of tar-like substances, which will adhere to the surface of SBC800 and lead to clogging of its porous structure. The difference in pore volume of different biochar is not obvious, so the pore volume has little effect on the adsorption capacity of biochar. Overall, the pore structure of biochar with larger specific surface area and smaller pore size is better, which can make it have stronger adsorption capacity, so it is hypothesized that SBC800 has stronger adsorption capacity for GSM (Table 2).

In this study, moss was used as a raw material to make biochar to adsorb odorous substances for the first time, and good results were achieved. Moss biochar from aquaculture by-products was used to remove the odor produced in aquaculture, which not only solved the problem of aquaculture by-products but also eliminated the impact of odor on the aquaculture environment, and the resources were recycled without waste. Including biochar production is simple, relatively low-cost, environmentally friendly, and will not produce secondary pollution in the water environment. However, there is no in-depth research on the elution and reapplication of biochar adsorbed with odorous substances.

With the development of human society and population growth, the demand for food and energy has increased dramatically, and humankind and the Earth's terrestrial ecosystems are under unprecedented pressure. Energy shortages and security issues have become global concerns, and these issues urgently require that resources be recycled in our development, production, and living to seek a sustainable path. Biomass, as a renewable energy source, is derived from living organism materials, most often referred to as plants or plant-derived materials. The recycling of biomass greatly improves the ecological environment and also helps optimize the energy structure (Xu & Chen 2020; Raj et al. 2022).

Nowadays, different techniques for the removal of odorous substances, such as adsorption using powdered activated carbon, coagulation, sedimentation, membrane filtration, flocculation, ozonation, electrodialysis, precipitation, co-precipitation, and multi-phase photocatalysis, have been applied to remove color, taste, or odor from water (Fotiou et al. 2016; Luo et al. 2016). However, each method has its own limitations.

Activated carbon has a large specific surface area, a well-developed microporous structure, and a strong adsorption capacity. Activated carbon will preferentially adsorb organic substances, so it has a good effect on the removal of olfactory substances, mainly through physical adsorption and microbial degradation (Zheng et al. 2023). The use of activated carbon adsorption of odor substances is feasible, but it is not easy to regenerate; the production of raw materials is complex with relatively high cost as a drawback (Mustapha et al. 2021). Advanced oxidation technology has been identified as the logical choice for the effective decomposition of odorous substances (Garrido-Cardenas et al. 2020). Photocatalyst semiconductors such as ZnO, WO₃, and Fe₂O₃ have been developed due to their wide range of technologically useful properties, such as good stability and ease of application, but may cause environmental problems (Banerjee et al. 2014). Some research investigated the photocatalytic activity of TiO₂ for the degradation and mineralization of GSM under solar and visible light irradiation (Fotiou et al. 2015). However, advanced oxidation is expensive and produces toxic sludge, and the process of removing odorants transfers contaminants from one phase to another without effective removal from the aqueous medium (Wang et al. 2023a). In study by Visentin et al. (2019) using vacuum UV to remove MIB and GSM, incomplete elimination of these compounds was observed due to the generation of oxidative by-products. Biological treatment is currently widely used in subsurface infiltration and biofilm methods. Ho et al. (2007) achieved 100% removal of GSM from surface waters in an Australian catchment through biofilters and conventional treatment processes, with biofilters dominating the removal effect. Biological treatment for the removal of GSM and 2-MIB has the advantages of cost reduction, by-products, and new pollution reduction, but the method is susceptible to a variety of factors such as water temperature, pH, and nutrients, and the treatment effect is not stable enough (McDowall et al. 2009; Luo et al. 2016).

To summarize, although there are many kinds of technologies to remove GSM, they all have various drawbacks, and to remove GSM, there is no method that has high removal efficiency, is environmentally friendly, and the raw materials are cheap and easy to obtain. The raw material of BC800 used in this study was originally an aquaculture by-product, which is difficult to be treated but cheap and easy to obtain and the process of making it is very simple, and it does not need too much modification and treatment to have a good adsorption effect on GSM, which can be up to 97.08%. Therefore, it is a better technology to use BC800 to adsorb GSM.

Moss is a type of algae that becomes a good adsorbent material when made into biochar, and other algae can also be made into biochar to remove other pollutants. Anthropogenic chemicals, such as heavy metals and organic pollutants, can frequently be detected in aquaculture products. Among these chemicals, cadmium (Cd²⁺) and polycyclic aromatic hydrocarbons (FLU, PHE, FLT, and PYR) are two of the most abundant chemicals in aquaculture products, which occasionally exceed safety limits in eastern China due to the presence of contaminants in aquaculture waters (Shin et al. 2013; Li et al. 2019a). Cd²⁺ in aquaculture ponds may come from road dust and runoff (Pal & Maiti 2018). Polycyclic aromatic hydrocarbons (PAHs), on the other hand, come from biomass combustion and vehicle emissions (Wang et al. 2010; Zeng et al. 2018). Wang et al. (2023b) found that biochar prepared from seabed moss was used to improve water quality in aquaculture ponds, and the removal efficiencies of seabed moss biochar for Cd²⁺ and PAHs were 49, 88, 90, 91, and 88%, respectively.

Chen et al. (2018) demonstrated that magnetically modified seashore biochar had a good adsorption effect on Cr. The modification introduced a large number of hydroxyl groups, and the –OH functional group was protonated to form the positively charged –OH₂⁺ functional group, which allowed Cr-containing anions to be adsorbed and migrated to the surface of γ-Fe²O³ by electrostatic interactions, and thus electrostatic interactions could be the main mechanism of Cr adsorption by the magnetic biochar (Chen et al. 2018). The adsorption mechanism of BC800 on GSM is both physical pore adsorption and chemical adsorption by hydrogen-bonded multimolecular layers.

Using aquaculture by-products of moss, Elodea, and sediment as raw materials, moss biochar, Elodea biochar, and sediment biochar were prepared by the high-temperature pyrolysis method, which broadened the source of biochar materials and screened the moss biochar calcined at 800 °C to be the most effective against the main odorous substance (GSM) in aquaculture. When treating a GSM-containing solution with a pH of 7 and a concentration of 300 ng L⁻¹, the adsorption efficiency of GSM reached 97.08% under the conditions of a BC800 dosage of 1.0 g, a temperature of 25 °C, and an adsorption time of 2 min.

The adsorption process of BC800 on GSM conforms to the quasi-primary and quasi-secondary kinetic models, and the Freundlich isotherm model has a good fit for the adsorption process of BC800 on GSM. It shows that the adsorption process of BC800 on GSM is multimolecular layer adsorption with both physical and chemical adsorption, which adsorbs GSM through electrostatic attraction, and surface complexation, and the adsorption process is exothermic.

This work was supported by the Center Public Interest Scientific Institution Basal Research Fund, the Freshwater Fisheries Research Center, CAFS (No. 2023JBFM03), China Agricultural Research System (CARS-46), Central Public-interest Scientific Institution Basal Research Fund, CAFS (NO. 2023TD18), Jiangsu Graduate Practice Innovation Project (SJCX23_0218), and Coordinated Promotion Project for Major Agricultural Technologies (2022-ZYXT-07).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.