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Article

Acute Toxicity Assessment of Textile Wastewater Treated with Pinus patula Biochar Using Daphnia pulex

by
Carolina Gallego-Ramírez
1,
Yuri García-Zapata
2,
Néstor Aguirre
2,3,
Edwin Chica
1 and
Ainhoa Rubio-Clemente
1,3,*
1
Grupo de Investigación Energía Alternativa (GEA), Facultad de Ingeniería, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín 050010, Colombia
2
Grupo de Investigación GEOLIMNA, Escuela Ambiental, Facultad de Ingeniería, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín 050010, Colombia
3
Escuela Ambiental, Facultad de Ingeniería, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín 050010, Colombia
*
Author to whom correspondence should be addressed.
Water 2025, 17(8), 1143; https://doi.org/10.3390/w17081143
Submission received: 14 March 2025 / Revised: 5 April 2025 / Accepted: 9 April 2025 / Published: 11 April 2025
(This article belongs to the Special Issue Emerging Micropollutants in Water and Wastewater: Recent Tendencies, Treatment Options and Perspectives)
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Abstract

The discharge of textile wastewater (TWW) into the environment releases multiple toxic substances that pose a significant threat to aquatic life. Most studies evaluating wastewater treatment efficiency focus on the removal of parameters, such as chemical oxygen demand (COD), total organic carbon (TOC), dissolved organic carbon (DOC), biochemical oxygen demand (BOD), and colour. One of the processes that has presented high efficiencies in the treatment of TWW is the use of biochar (BC) as an adsorbing material. BC has shown a high ability to remove complex organic substances from water since it is able to decrease the content of COD, TOC, and DOC. However, the toxicity of treated effluents has not been widely studied. In this regard, it is essential to focus not only on the efficiency of treatments in removing organic matter but also on their ability to reduce WW toxicity. This research evaluates the acute toxicity of real TWW treated with Pinus patula BC by using Daphnia pulex as a sentinel species. For this purpose, D. pulex individuals were exposed to TWW and BC-treated TWW for 48 h, with mortality defined as the absence of movement in the limbs and antennas. It was found that although the treatment with P. patula BC for 120 min eliminated 72.8% of the initial DOC under optimal conditions (pH 3 and 13.5 g/L BC dose), the textile effluent remained toxic, inducing 85.7% and 71.4% mortality rates on D. pulex for 100% (v/v) and 50% (v/v) dilutions. Despite the increase in the survival rate of D. pulex individuals due to the protective effect achieved by the constituents contained in the reconstituted 50% (v/v) samples, these findings emphasize the necessity of conducting toxicity studies before considering the discharge of TWW effluents after having been treated.

1. Introduction

The textile industry is one of the oldest and largest industries that has contributed significantly to the development of the world economy [1]. The processing of 1 ton of textiles can use more than 100 to 200 m3 of water, of which 80–90% is discharged as wastewater (WW) [2]. In this regard, the textile industry is known for the discharge of large volumes of WW, being the second largest source of water pollution worldwide [1]. Textile wastewater (TWW) is composed of dyes, inorganic salts, heavy metals, surfactants, additives, and refractory organic matter [2,3]. Therefore, the discharge of TWW to water bodies without any treatment or with an inefficient treatment can result in the pollution and degradation of the environment due to the release of compounds toxic for living organisms [3].
The textile industry uses hundreds of thousands of synthetic dyes to impart colour to textile fabrics [4]. Dyes can be classified according to the functional groups present in their chromophores and the method of application. The chromophores in dye molecules can include azo, acridine, anthraquinone, diphenylmethane, oxazine, indigoid phthalocyanine, xanthene, thiazine, and triphenylmethane groups. Therefore, dyes can be classified as anionic (acid, reactive, and direct), cationic (basic), non-ionic (disperse and vat), azoic, or sulphur dyes. Anionic and cationic dyes are soluble in water, while non-ionic dyes are insoluble, whose application involves raising the water temperature to increase their solubility [5]. Most dyes used on textile fibres are azo dyes, such as Congo Red, Acid Red, Methyl Orange, Acid Orange 7, Reactive Black 5, Malachite Green (MG), Direct Blue 15, Disperse Orange 37, and Acid Red 88. Azo dyes are characterized by azo bonds (N=N), which are linked to various chemical groups, including nitro, amino, methyl, sulphonate, hydroxyl, and chlorine groups, giving the dye molecule a structural diversity that enables a wide range of colours. Additionally, the azo bond increases the dye’s affinity for textiles, along with the dye solubility, which prevents the dye from easily fading from the fibres. The widespread use of azo dyes is due to their functional and structural properties [4,6]. On the other hand, indigoid dye, especially Indigo Carmine (IC), is another class of dye extensively used in the textile industry. The chromophore of indigoid dyes is a conjugated system of a C=C bond replaced by two amine (NH2) and two carbonyl (C=O) groups. IC is widely utilized in the dyeing of denim due to its high solubility and affinity for fibres [7].
Synthetic and complex dyes used in the textile industry are responsible for the persistent colour, total dissolved solids (TDSs), chemical oxygen demand (COD), and total suspended solids (TSSs) in TWW [4], leading to toxic, mutagenic, and carcinogenic effects on mammalian cells, plants, molluscs, fishes, and microorganisms [5]. Hence, textile effluents must undergo treatment prior to their environmental discharge.
Conventional treatments, including coagulation–flocculation, oxidation, and biological degradation have been applied to treat TWW [5]. Nevertheless, these processes present limitations like low efficiencies, high operational costs, and the production of sludge [8]. Consequently, alternative treatments offering high efficiencies, low maintenance and operational costs, and easy application are required to reduce both organic matter and toxicity in TWW [9].
Biochar (BC) adsorption has emerged as a promising alternative for TWW treatment. BC is a carbonaceous material derived from the pyrolysis, gasification, torrefaction, or hydrothermal carbonization of residual biomass, including animal manure, sewage sludge, industrial and agricultural by-products, invasive plants, and wood waste [10]. The wood industry, in particular, produces significant amounts of residues. For instance, during the processing of Pinus patula wood, which is one of the most commonly used forestry species due to its high quality and rapid growth in tropical regions, only 35% of the harvested wood is utilized in the final product. Consequently, approximately 65% of P. patula wood is discarded [11]. This residue presents an environmental challenge, as it is often disposed of through incineration, resulting in air quality deterioration and wildfire risk [12]. Therefore, the conversion of P. patula wood waste into BC can be regarded as an environmentally friendly alternative, since it can be subsequently applied for pollutant removal from WW, contributing to sustainable development and environmental remediation [11,13,14].
The utilization of BC in WW treatment is related to its physicochemical properties (e.g., oxygen-functional groups, high surface area, and porous structure) [15], allowing it to be considered as the most optimal adsorbing material due to its ease of operation, cost effectiveness, high removal efficiency, easy availability, environmental friendliness, and regeneration capacity [15,16]. Regarding the treatment of TWW with BC, studies have mainly focused on the evaluation of BC adsorption capacity on synthetic WW. Indeed, Hong et al. [17] investigated the removal of Methylene Blue (MB) from simulated WW using pine sawdust BC. The referred authors found that BC was able to eliminate 97.25% of MB. This high removal efficiency was attributed to chemical interactions between the BC surface and the dye molecules. Likewise, Cavali et al. [18] analyzed the efficiency of BC produced from sawdust and sewage sludge to eliminate the same dye. The water matrix also consisted of a synthetic WW, and a removal efficiency of 94% was found. In turn, Nnadozie and Ajibade [19] evaluated the removal of IC by using BC derived from Chromolaena odorata, reaching a removal efficiency of 94.7%.
It is important to note that on simulated WW matrices, high efficiencies are expected to be reached, since dye molecules do not compete with other matrix constituents to take up the BC adsorption active sites. In real WW, however, background matrix components can interfere with adsorption effectiveness. Competing substances may occupy active sites intended for target pollutants, often necessitating combined treatment approaches incorporating BC adsorption. In this regard, research evaluating treatment efficiency on real TWW has demonstrated that açaí-derived BC, when combined with coagulation–flocculation, can reduce its biochemical oxygen demand (BOD) by up to 80% and COD by 48.8% [20].
Although research on BC for TWW treatment has expanded recently, significant knowledge gaps remain. Current limitations include challenges in industrial scaling, feedstock variability affecting BC physicochemical properties, spent adsorbent disposal, adsorption capacity optimization for real WW, and process safety assessment [21,22,23]. To ensure the successful scaling and practical application of BC adsorption, these gaps must be addressed through mechanistic studies and comprehensive performance evaluations.
Concerning the adsorption process’s safety, water toxicity analyses must be conducted to assure an accurate risk assessment and the discharge of non-toxic effluents into the environment [24]. Toxicity analysis generally involves the measurement of contaminant levels and the assessment of damage to freshwater and/or marine organisms to characterize the risk to the aquatic environment [25]. Acute and chronic toxicity tests are used to monitor the toxicity of water to determine the adverse effects that occur on aquatic organisms. In acute toxicity tests, organisms are exposed for a period ranging from 24 to 96 h to determine whether immobilization or mortality is induced because of exposure to the WW. Meanwhile, in chronic toxicity tests, organisms are exposed for longer periods (i.e., 21 days) to discern sub-lethal effects, such as growth, reproduction, and survival [26]. Acute and chronic tests are carried out using a sentinel species, which is considered as an organism whose sensitivity enables it to detect modifications in environmental conditions and ecosystem structure so that a change in its normal behaviour can be detected [27]. The use of sentinel organisms in toxicity studies allows the integration of the diversity of bioavailable pollutants contained in WW into the assessment of effluent toxicity [28].
Fish, crustaceans, bivalves, and primary producers like algae and plants are used as sentinel species [25]. Daphnia pulex is one of the most commonly utilized species in toxicological studies due to its high sensitivity to pollutants, its parthenogenetic reproductive rate, which allows the use of individuals with the same genetic characteristics, and its ease of culture at a laboratory scale [29]. D. pulex can be found in standing freshwater from small ponds to large lakes. As a zooplankton species, D. pulex plays a crucial role in freshwater food chains, making it ecologically significant, since it is the main primary consumer filer, feeding on unicellular algae and other particles. By consuming phytoplankton, D. pulex helps regulate algae populations and prevent algal blooms that can lead to water oxygen depletion. Additionally, D. pulex is prey for fish and various invertebrate predators [30]. As observed in Figure 1, zooplankton, as a primary consumer, is at a low level in the food chain; hence, adverse effects on zooplanktonic organisms can lead to effects at higher levels in the food chain, even causing its collapse [24].
Given the ecological importance of D. pulex and the need to determine the safety of BC-treated TWW and to start filling knowledge gaps to increase the applicability of BC at an industrial scale, focusing on real TWW, this study evaluates the acute toxicity of Pinus patula BC-treated TWW using D. pulex. The purpose of this research is to discern the effectiveness of P. patula-derived BC in mitigating the toxicity of the textile effluent while removing organic matter so that a comprehensive assessment of BC’s potential to simultaneously reduce both the organic load and overall toxicity in real TWW is provided.

2. Materials and Methods

2.1. Chemicals, Reactants, and Textile Wastewater (TWW) Collection

For pH adjustment, hydrochloric acid (HCl) and sodium hydroxide (NaOH) were provided by VWR chemicals (Radnor, PA, USA) and Sigma Aldrich (Darmstadt, Germany) with a purity of 37.20% and >99%, respectively. TWW was obtained from a company specialized in denim dyeing and located in Colombia. TWW samples were manually collected in opaque polyethylene containers and stored at 4 °C in the dark until analysis to keep their physicochemical and biological characteristics. TWW was sampled directly from the textile production process and did not undergo any treatment prior to the BC adsorption process performance.

2.2. Pinus patula-Derived Biochar (BC) Production and Characterization

P. patula wood pellets were supplied by a Colombian sawmill. To produce the BC, the P. patula wood pellets were gasified in a fixed-bed top-lit updraft reactor at atmospheric pressure using air as the gasification agent. The gasification process reached a maximum temperature of 700 °C [14]. When BC is produced from a primary feedstock, like wood, at a high temperature, it presents a high surface area and a large pore volume due to the release of volatile and aliphatic organic compounds [31], resulting in a high adsorption capacity. After production, the obtained P. patula BC was crushed and sieved until a particle size of 300–450 µm was obtained. The BC derived from P. patula was stored in plastic zip-lock bags before use. Further information regarding the production of the BC can be found in Rubio-Clemente et al. [32] and Gutiérrez et al. [14]. The P. patula wood pellets and BC were characterized via the BET nitrogen (N2) adsorption method, thermogravimetric analysis (TGA) to conduct proximate and ultimate analyses, and pH at the point of zero charge (pHpzc). Additionally, scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) were performed, as reported by Gutiérrez et al. [14] and Rubio-Clemente et al. [32].

2.3. TWW Characterization and Batch Adsorption Studies

The TWW properties were determined by measuring total organic carbon (TOC), COD, BOD after 5 d of incubation (BOD5), conductivity, real and apparent colour, pH, dissolved organic carbon (DOC), and temperature.
On the other hand, adsorption studies were carried out in triplicate to evaluate the efficiency of P. patula BC to reduce the initial DOC of the TWW. In a previous study, the optimal operating conditions for removing IC from water by using the P. patula-derived BC utilized here were evaluated. The optimal pH and BC dose were found to be 3 and 13.5 g/L, respectively [33]. Since IC is the primary dye used in denim manufacturing [7], the present study employed the operating conditions for real TWW treatment as previously described [33,34].
For the batch adsorption studies, a suspension of 200 mL of real TWW and 13.5 g/L of P. patula BC was added to a 0.6 L glass container. The pH of the solution was adjusted to 3 by using HCl 1 M. To ensure interactions between the TWW constituents and the BC, the solution was stirred by placing the container with a stirring bar inside in a stirring plate. Aliquots of 20 mL were taken from the suspension at 0, 5, 15, 30, 60, and 120 min and were filtered using a vacuum pump with cellulose filters of a 0.45 µm pore diameter to remove the particles of P. patula-derived BC. Afterwards, the DOC of each sample was measured using a TOC analyzer (Shimadzu Corporation, Columbia, MD, USA). The effectiveness of the process in terms of the removal of DOC was calculated using Equation (1), where E is the removal efficiency (%), DOC0 (mgC/L) is the DOC of the TWW before the treatment, and DOCF (mgC/L) is the DOC of the TWW treated with P. patula-derived BC.
E = D O C 0 − D O C F D O C 0 × 100

2.4. Acute Toxicity Assessment Using D. pulex

D. pulex individuals were sampled from a water reservoir located in Antioquia, Colombia. In the laboratory, the D. pulex individuals were cultured in reconstituted water, which was prepared by adding magnesium sulphate (MgSO4), sodium hydrogen carbonate (NaHCO3), potassium chloride (KCl), and calcium sulphate dihydrate (CaSO4·2H2O) to deionized water. The solution was stirred until full homogenization, and the pH was adjusted to a range of 7.6–8.0. Once the desired pH was achieved, the solution was aerated for 24 h [35]. The culture of D. pulex was maintained at 20 °C under a 12:12 h (light/dark) photoperiod. The organisms were fed with Spirulina sp. every other day (EOD). The culture was EOD-cleaned, and a water change was performed every 8 days.
Different solutions were prepared for testing the acute toxicity, as shown in Figure 2. The control group consisted of reconstituted water, while the raw textile effluent was represented by a sample collected at 0 min of BC treatment. In addition, two dilutions were evaluated: 100 and 50% (v/v). For the dilution of 100% (v/v), five treated TWW samples corresponding to different contact times (5, 15, 30, 60, and 120 min) were analyzed. To simulate environmental conditions, 50% (v/v) dilutions were prepared for both the raw and treated effluent samples. Each test solution was placed in clear containers, with 20 mL per container. For the 50% (v/v) dilution, 10 mL of raw or treated TWW was mixed with 10 mL of reconstituted water to achieve the desired dilution ratio. After the solutions were prepared and placed in the clear containers, 5 neonates of D. pulex were transferred. The number of neonates per test was chosen to ensure the use of the smallest possible number of individuals while maintaining statistical adequacy [29,36,37]. The temperature during the tests was 20 °C. Mortality was recorded after 48 h of exposure and was determined as the absence of antennae and limb mobility. Each test was conducted in triplicate, and mortality was calculated according to Equation (2), where M (%) is the percentage of mortality induced by the solutions, Ninitial individuals corresponds to the initial number of D. pulex individuals added to each solution (5), and Nliviing individuals refers to the number of surviving individuals after 48 h of exposure [38].
M = N i n i t i a l   i n d i v i d u a l s − N l i v i n g   i n d i v i d u a l s N i n i t i a l   i n d i v i d u a l s × 100

3. Results and Discussion

3.1. P. patula-Derived BC Characterization

P. patula wood pellets and their resulting BC were characterized by measuring their pore volume and surface area through BET N2 adsorption. Additionally, a TGA was performed, as reported in Table 1. In this Table, it can be observed that the gasification process increased the surface area of the wood pellets. High-temperature BC production led to the carbonization of the organic compounds contained in the biomass, forming micro- and meso-pores that enhanced the BC surface area and adsorption capacity [39]. The increase in the pore volume indicates that P. patula-derived BC has an improved potential for removing pollutants from water by retaining molecules within its porous structure [40].
Regarding the ultimate analysis, a decrease in hydrogen (H) and oxygen (O) contents after gasification was achieved, which is attributed to the loss or modification of functional groups, such as carbonyl (–CO), hydroxyl (–OH), aliphatic (–CH2–), and carboxyl (–COOH) groups, as well as to the degradation of the cellulose and hemicellulose of the biomass [14,41]. The calculated H/C and O/C ratios decreased in the generated BC compared to the original wood pellets. This reduction in the O/C and H/C ratios indicates that the BC possesses a hydrophobic surface and an aromatic structure [42], characteristics that are inherent to lignin-rich biomasses like P. patula wood [43]. BC with high aromaticity results in stronger electron accepting–giving interactions between the π electrons on the BC surface and the organic molecules contained in TWW. In turn, the decrease in the O/C ratio reveals a reduction in BC hydrophilicity and polarity, influencing the interaction of the BC surface with polar organic substances [44].
Volatile material (VM) content was also determined by proximate analysis for both the biomass and BC. The gasification process resulted in the release of VM from the wood pellets, as evidenced by a decrease in the VM content (Table 1). This VM loss is associated with the formation of pores and an increase in the surface area, which in turn enhances the adsorption capacity of the BC [32,45]. Finally, the pHpzc of the P. patula-derived BC resulted to be 6. Hence, at a solution pH of 6, the surface of BC presents a neutral charge. At pH < 6, the BC surface would be protonated, resulting in a positively charged surface and favouring the removal of negatively charged molecules from water though electrostatic attraction. In contrast, at a pH value above 6, the surface of P. patula BC would be protonated due to a high concentration of hydroxyl ions (OH−), improving the removal of positively charged molecules contained in water [46].
On the other hand, the SEM analysis reported in Gutierrez et al. [14] revealed morphological changes in the biomass due to gasification. Indeed, P. patula wood pellets exhibit lignin agglomerations on their surfaces, which is consistent with the high lignin content of the raw biomass. In contrast, P. patula BC exhibits a porous structure resulting from the release of VM during gasification, which is expected to enhance pollutant removal from water [47]. In turn, the FTIR spectra indicated the loss of several functional groups following gasification; this change is attributed to the release of volatile substances, dehydration, and the thermal degradation of organic bonds. For more detailed information on the characterization of P. patula-derived BC, including the FTIR spectra and SEM images, refer to Gutiérrez et al. [14].

3.2. Efficiency of P. patula-Derived BC in the Removal of DOC from TWW

As observed in Table 2, the TWW had a pH close to neutrality (6.4), an intense blue colour due to the content of the IC dye, and a conductivity of 2.4 mS/cm, which was related to the use of salts during the dyeing process to increase the fixation of the dyes on fibres [48]. TWW also presented a biodegradability index (BOD5/COD) of 0.35, indicating that physicochemical processes, including adsorption, are more suitable for its treatment [49].
As presented in Figure 3, the maximum removal efficiency by using a dose of 13.5 g/L of P. patula-derived BC and a solution pH of 3 was obtained at 120 min of treatment time, achieving a DOC removal from 124.5 mgC/L to 28.86 mgC/L. These findings suggest that P. patula-derived BC was able to remove 76.82% of the initial dissolved organic substances contained in the TWW effluent [50]. The measurement of parameters related to the concentration of organic substances in WW is linked to the ecological and chemical assessment of water quality due to the complexity and toxicity of the organic substances that constitute TWW [51]. Therefore, a decrease in the initial DOC of TWW should represent a decrease in the concentration of toxic organic substances contained in the textile effluent. Considering that the TWW used originated from a denim-dyeing plant, IC was identified as the predominant dye [7,52]. As an anionic dye, IC has a negative charge in aqueous solution, which increases its affinity for positively charged adsorbent surfaces. At a solution pH of 3, P. patula-derived BC exhibits a positively charged surface due to protonation from the high concentration of H+ and its pHpzc (6) [33,53]. Therefore, under these acid conditions, IC molecules are attracted to the positively charged BC surface, facilitating their removal and resulting in a reduction in the DOC contained in TWW.
The removal of dyes from the TWW can be attributed to multiple mechanisms. Primarily, electrostatic attraction occurs between the negatively charged IC molecules and the BC surface. Additionally, π–π electron interactions take place between the π electrons on the P. patula BC surface and the electron cloud of the benzene rings in the dye molecule. The high removal rate of DOC can also be related to the simultaneous elimination of other negatively charged organic molecules through various processes, including electrostatic attraction, hydrogen bonding, pore filling due to the high surface area of the P. patula-derived BC, and π–π electron interactions [54,55].
Different studies have demonstrated the efficiency of BC in treating WW due to the physicochemical characteristics of BC [56]. Recent research has focused on critical knowledge gaps in BC-based water treatment, particularly regarding real WW applications, spent BC disposal, and cost-effectiveness analyses. For economic assessments, BC is frequently benchmarked against activated carbon, the current industry-standard adsorbent. Studies show BC treatment costs range from 100 to 600 USD/ton, depending on production methods, while activated carbon typically costs 800–1000 USD/ton. This significant cost advantage, combined with BC’s stability and regeneration capacity through multiple adsorption–desorption cycles, enhances its economic viability [57,58].
Techno-economic analyses have demonstrated BC’s dual benefits of high pollutant removal efficiency and cost-effectiveness [16]. Nevertheless, challenges remain in industrial scalability, adsorption capacity optimization, effluent toxicity assessment, and feedstock variability [22]. Addressing these limitations is crucial for advancing BC implementation in full-scale TWW treatments.

3.3. Acute Toxicity of TWW Treated with P. patula-Derived BC Using D. pulex

D. pulex was used as a sentinel organism to assess the toxicity of TWW treated with P. patula-derived BC. The control for the toxicity test was conducted by exposing D. pulex to reconstituted water. D. pulex individuals were also exposed to a raw textile effluent, consisting of an untreated effluent, and BC-treated textile effluents at 5, 15, 30, 60, and 120 min to evaluate the changes in toxicity with increases in the treatment time. To analyze a more realistic scenario, the toxicity at 50% dilution (v/v) was also measured for the raw and the treated TWW.
For 100% dilution (v/v), the normalized mortality rate is shown in Figure 4. It can be observed that the textile effluent without P. patula-derived BC treatment (at 0 min of treatment) caused a mortality of one, meaning that the TWW resulted in an acute toxicity in 100% of the exposed D. pulex individuals. The mortality induced by the exposure to the raw textile effluent is related to the complexity of the effluent. As stated above, textile effluents generally contain high concentrations of toxic dyes, derivatives, and solids that are detrimental to aquatic life [59]. The concentration of salts in the TWW (conductivity = 2.4 mS/cm) may also be responsible for the mortality rate caused by the untreated TWW. It has been reported that water with a high concentration of salts can induce toxic effects on D. pulex due to direct osmotic effects [29]. Table 3 summarizes several studies that have assessed the toxicity of TWW using Daphnia sp. as the sentinel organism. These studies indicate that untreated TWW can generate harmful effects on aquatic organisms due to their toxic properties. Similar results were obtained by Nallasamy et al. [29], who evaluated the toxicity of a textile effluent using zebrafish (Danio rerio) individuals. It was found that the textile effluent was highly toxic to zebrafish, with the survival rate decreasing to 90% after 2 days of exposure and to 30% after an exposure time of 5 days. In turn, Gil-Pavas et al. [60] assessed the toxicity of TWW on the Artemia salina crustacean and found that the effluent caused an acute toxicity in 100% of the exposed individuals. This high mortality rate was attributed to the high concentration of dyes in the textile effluent and other raw materials employed during the production process in the textile plants. These results suggest that untreated WWT is highly toxic, as acute effects in aquatic organisms can be caused.
As illustrated in Figure 4, the mortality rate remained at 100% when TWW was treated with P. patula-derived BC for 5, 15, and 30 min. This suggests that BC was not effective in reducing the toxicity of the treated textile effluent under the experimental domain tested. The high toxicity rates found can be attributed to the lixiviation of organic compounds from the BC surface. It has been suggested that organic compounds can migrate from the BC surface into the solution and produce a combined and synergistic toxic effect with the components of the TWW effluent, inducing an acute toxicity to D. pulex individuals [65]. When the treatment time was extended to 60 min, the mortality rate decreased to 0.857 (85.7% of the individuals were affected by the TWW-treated effluent) and remained at this level after increasing the treatment time to 120 min.
Even though the toxicity rate decreased when the P. patula-derived BC treatment time increased to 120 min, the treated-textile effluents still presented a high toxicity rate on D. pulex individuals. This elevated toxicity likely resulted from both the inadequate removal of toxic compounds from the TWW and the suboptimal pH conditions during treatment. Here, the treatment of TWW with P. patula-derived BC was carried out at a pH of 3, and the samples were not neutralized before conducting the acute toxicity tests. It is highlighted that the optimal pH for the optimal survival of D. pulex organisms is between 6.5 and 9.5. Hence, pH values below or above the optimal range can cause the mortality of individuals since the environmental conditions are not adequate [66]. To avoid the effect of pH on D. pulex, 50% (v/v) dilutions were prepared for the aliquots withdrawn within the time interval of this study. It is important to note that when acid effluents are discharged into water bodies with an effective mixing and buffering capacity, the pH of the WW can be gradually neutralized. Consequently, aquatic organisms, like D. pulex, are protected from rapid pH fluctuations, thereby reducing the adverse effects on their survival. Water bodies containing bicarbonates (HCO3−), carbonates (CO32−), and hydroxides (OH−) exhibit strong buffering capacities that maintain a stable pH. In this regard, following pollutant discharge, the pH returns to its prior equilibrium within an optimal range for aquatic life [67].
The normalized acute toxicity results for the 50% (v/v) dilution are presented in Figure 5. For this dilution, the mortality rate at 0, 5, and 30 min was one. Nonetheless, the samples taken at 120 min and diluted with reconstituted water showed a lower mortality rate compared to the same samples for the 100% (v/v) dilution (i.e., 0.714 and 0.857, respectively). The normalized mortality for the dilution of 50% (v/v) at 120 min indicated that the diluted and treated TWW caused acute toxicity in 71.4% of the exposed individuals. The decrease in the toxicity rate for the 50% (v/v) dilution samples was attributed to a protective effect of the constituents of the reconstituted water against the toxic constituents of the TWW that were not removed [29]. Considering that water bodies with strong buffering capacities can mitigate the toxic effects of pollutant discharges, it can be anticipated that when P. patula BC-treated TWW is released into the environment, the pH will be neutralized. This neutralization is expected to reduce the mortality rate of aquatic organisms exposed to the BC-treated TWW, as was identified in the findings obtained for the 50% (v/v) dilution [67].
On the other hand, according to the literature reports, studies aimed at evaluating the toxicity of treated textile effluents have shown that a single treatment is usually not enough to remove all the toxic substances present in TWW [59]. As a matter of fact, Gil-Pavas et al. [68] used electrocoagulation to treat TWW. In their acute toxicity study using Artemia salina, the referred authors found that even though the COD met the Colombian discharge limit for textile effluents (COD < 400 mgO2/L) at 60 min of treatment, the treated textile effluent was still highly toxic, inducing 100% mortality in Artemia salina. Therefore, the application of several sequential processes was required to achieve the desired water quality, reduce toxicity to aquatic organisms, and achieve a more conclusive remediation result [21,59]. Gil-Pavas and Correa-Sánchez [49] evaluated this approach by means of the treatment of TWW using electro-oxidation followed by adsorption with activated carbon. Electro-oxidation alone did not reduce acute toxicity towards Artemia salina, obtaining a mortality rate of 100%. Due to this high toxicity, the effluent underwent a second treatment with activated carbon. This dual treatment significantly reduced the acute toxicity, lowering mortality to 20% [49]. In another study, Gil-Pavas et al. [60] investigated the removal of toxicity from TWW using a combination of three treatments: electrocoagulation, electro-oxidation, and adsorption with activated carbon. This sequential treatment effectively reduced the toxicity of the WW on Artemia salina, achieving a mortality rate of 0%. These results demonstrate that the combination of several treatment technologies can eliminate toxic substances from TWW effluents, reducing the hazard of these effluents to aquatic life.
Beyond combining BC with other treatments to improve removal efficiency and reduce effluent toxicity, BC can be directly modified to enhance its adsorption capacity. Modified BC demonstrates a superior performance compared to pristine BC in eliminating high-concentration contaminants from WW, yielding less toxic effluents [22]. BC modifications include the support of additional materials, elements, or compounds on its surface, enhancing its surface properties, such as pore structure and functional groups [22,56,69]. BC can be modified using acids and bases, including HCl, sulphuric acid (H2SO4), nitric acid (HNO3), phosphoric acid (H2PO4), potassium hydroxide (KOH), and NaOH. Acid and alkaline modifications are carried out to increase BC surface area and oxygen functional groups and remove ash and impurities that may block active sites. In turn, salt modifications are also employed to load metals like iron (Fe), copper (Cu), manganese (Mn), and magnesium (Mg) on BC, thereby improving the adsorption of specific substances and enabling BC to function as a catalyst [56]. The physical activation of BC consists of its exposure to carbon dioxide (CO2), steam, hydrogen (H2), air, oxygen (O2), and ammonia (NH3). In physical activations, the surface area of BC is improved, closed micropores are open, and mesopores are formed by enlarging existing pores [69]. Additionally, BC can be modified with semiconductors such as zinc oxide (ZnO) and titanium dioxide (TiO2) to serve as a catalyst in advanced oxidation processes (AOPs), promoting the degradation of pollutants into simpler molecules [70].
Studies evaluating the adsorption capacity of modified BC on synthetic WW have demonstrated that this modified adsorbing material can reduce the toxicity of dyes. In a study conducted by Ullah et al. [40], BC was modified with cooper oxide (CuO) and Mn to use it as a photocatalyst for Eriochrome Black T (EBT) elimination in synthetic WW. They found that after the treatment, the toxicity of EBT towards Daphnia and green algae decreased, leading to the production of a less toxic effluent. Furthermore, in the study carried out by Das and coworkers [71], a BC that was previously activated with NaOH was used to remove MG dye from synthetic WW. The process reached a removal efficiency of 90% and it was found that the effluent obtained did not produce toxic effects on Vigna mungo seeds, since an inhibition of seed growth was not observed [71]. These findings suggest that the modification of BC can be effective in the removal of toxicity from TWW. However, further studies using real TWW are needed to assess the performance of modified BC in real-world conditions and determine whether the observed toxicity reduction can be replicated in authentic TWW.

4. Conclusions

BC derived from P. patula wood pellet gasification was shown to be an efficient alternative material in the elimination of DOC from textile effluents. At 120 min of treatment, P. patula-derived BC was able to eliminate 76.82% of the initial organic compounds that contributed to the DOC of the TWW under a pH of 3 and a BC dose of 13.5 g/L. Despite the high removal percentage of DOC, the TWW treated with P. patula-derived BC remained highly toxic to D. pulex, since at 120 min, the normalized mortality rate was 0.857 (85.7%) for the dilution of 100% (v/v). However, for the dilution of 50% (v/v), a mortality reduction in D. pulex individuals of 0.714 (71.4%) was observed for this treatment time. In this regard, diluting the treated textile effluent to 50% (v/v) with reconstituted water showed a protective effect on D. pulex individuals.
Reconstituted water helped reduce the effects of residual toxic substances and moderate the pH value of the water. This dilution effect can simulate the conditions treated TWW may experience when discharged into a water body, where mortality rates associated with an acid pH and residual contaminants within the WW effluent are reduced. Nevertheless, to minimize possible risks to aquatic life, multiple sequential treatment processes are recommended. Furthermore, the efficiency of modified BC in reducing toxicity in real WW must also be evaluated to determine whether the results observed in synthetic WW can be replicated in real TWW.
These findings provide valuable insights for future research in the field, where complementary processes and BC modification practises can be integrated to improve overall treatment efficiency and ensure the survival of aquatic organisms.

Author Contributions

C.G.-R.: Investigation, conceptualization, writing—original draft preparation, methodology, and formal analysis. Y.G.-Z.: Conceptualization, methodology, formal analysis, and writing—review and editing. N.A.: Conceptualization, methodology, writing—review and editing, supervision, and resources. E.C.: Conceptualization, writing—original draft preparation, writing—review and editing, resources, and supervision. A.R.-C.: Conceptualization, writing—original draft preparation, methodology, writing—review and editing, formal analysis, supervision, resources, funding acquisition, and project administration. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to gratefully thank Project No. 2023-62610 of the University of Antioquia for financial support. Additionally, the financial support provided by the Universidad de Antioquia (Estrategia de Sostenibilidad 2023. ES84230042) is acknowledged.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 17 01143 g001
Figure 1. Freshwater food chain.
Figure 1. Freshwater food chain.
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Figure 2. Methodology followed for conducting the acute toxicity tests.
Figure 2. Methodology followed for conducting the acute toxicity tests.
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Figure 3. Dissolved organic carbon (DOC) removal from textile wastewater (TWW) using Pinus patula-derived biochar. Operational conditions: TWW volume = 200 mL; biochar dose = 13.5 g/L; pH = 3.
Figure 3. Dissolved organic carbon (DOC) removal from textile wastewater (TWW) using Pinus patula-derived biochar. Operational conditions: TWW volume = 200 mL; biochar dose = 13.5 g/L; pH = 3.
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Figure 4. Normalized mortality of D. pulex induced by TWW treated with P. patula-derived biochar at different treatment times (100% v/v). Operational conditions: Control = 20 mL of reconstituted water; TWW volume = 20 mL; D. pulex individuals per test: 5; T = 20 °C; exposure time = 48 h.
Figure 4. Normalized mortality of D. pulex induced by TWW treated with P. patula-derived biochar at different treatment times (100% v/v). Operational conditions: Control = 20 mL of reconstituted water; TWW volume = 20 mL; D. pulex individuals per test: 5; T = 20 °C; exposure time = 48 h.
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Figure 5. Normalized mortality of D. pulex induced by TWW treated with P. patula-derived biochar at different treatment times (50% v/v). Operational conditions: Control = 20 mL of reconstituted water; TWW volume = 10 mL; reconstituted water volume = 10 mL; D. pulex individuals per test: 5; T = 20 °C; exposure time = 48 h.
Figure 5. Normalized mortality of D. pulex induced by TWW treated with P. patula-derived biochar at different treatment times (50% v/v). Operational conditions: Control = 20 mL of reconstituted water; TWW volume = 10 mL; reconstituted water volume = 10 mL; D. pulex individuals per test: 5; T = 20 °C; exposure time = 48 h.
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Table 1. Physicochemical properties of Pinus patula wood pellets and BC.
Table 1. Physicochemical properties of Pinus patula wood pellets and BC.
PropertyUnitsWood PelletsBC
Surface area (BET)m2/g1.16367.33
Pore volumecm3/g0.00060.20
Nwt%0.020.19
Owt%47.280.9
Hwt%5.690.97
Cwt%47.0197.94
H/C-1.450.12
O/C-0.750.01
Volatile material (VM)wt%84.6420.59
pHpzc--6
Table 2. Textile wastewater (TWW) initial characterization.
Table 2. Textile wastewater (TWW) initial characterization.
ParameterUnitValue
Chemical oxygen demand (COD)mgO2/L630.3
5 d biochemical oxygen demand (BOD5)mgO2/L222.2
BOD5/COD-0.35
Temperature°C24.9
True colourPt-Co201
Apparent colourPt-Co>90
Total organic carbon (TOC)mgC/L217.9
Dissolved organic carbon (DOC)mgC/L124.5
ConductivitymS/cm2.4
pHpH units6.4
Table 3. Toxicity studies of TWW on Daphnia sp.
Table 3. Toxicity studies of TWW on Daphnia sp.
SpeciesExperimental ConditionsResultsReference
D. magnat = 48 h
Number of neonates per sample = 5
Photoperiod: 12:12
T = 25 °C
Dilutions of the TWW were performed with reconstituted water
−
The TWW concentration causing death in 50% of the tested organisms was 29.7%.
−
TWW was classified as toxic for aquatic organisms.
[37]
D. magnat = 24 h
Number of neonates per sample = 5
Photoperiod: 12:12
T = 25 °C
Dilutions of the TWW were performed with reconstituted water (100, 50, 25, 12.5, 6.2, and 3.1%)
−
The TWW concentration causing immobilization in 50% of the tested organisms was 44.8%.
−
The high toxicity was attributed to the dye molecules and the heavy metals used to increase dye fixation to fibres.
[61]
D. magnat = 48 h
Number of neonates per sample = 10
Photoperiod: 16:8
Dilutions of the TWW were performed with reconstituted water
−
The TWW concentration causing death in 50% of the tested organisms was 20.6% (v/v).
−
TWW was classified as toxic for aquatic organisms.
[62]
D. magnat = 48 h
Number of neonates per sample = 5
Photoperiod: 16:8
Dilutions of the TWW were performed with reconstituted water (100, 66.67, 44.44, 29.63, 19.75, 13.17, and 8.78%)
−
The TWW concentration causing immobilization in 50% of the tested organisms was 24.2%.
[63]
D. magnat = 48 h
Number of neonates per sample = 5
Photoperiod: 16:8
T = 20 °C
Dilutions of the TWW were performed with reconstituted water (100–1%)
−
The TWW concentration causing immobilization in 50% of the tested organisms was 53.82%.
−
TWW was classified as toxic for aquatic organisms due to its complex matrix.
[64]
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MDPI and ACS Style

Gallego-Ramírez, C.; García-Zapata, Y.; Aguirre, N.; Chica, E.; Rubio-Clemente, A. Acute Toxicity Assessment of Textile Wastewater Treated with Pinus patula Biochar Using Daphnia pulex. Water 2025, 17, 1143. https://doi.org/10.3390/w17081143

AMA Style

Gallego-Ramírez C, García-Zapata Y, Aguirre N, Chica E, Rubio-Clemente A. Acute Toxicity Assessment of Textile Wastewater Treated with Pinus patula Biochar Using Daphnia pulex. Water. 2025; 17(8):1143. https://doi.org/10.3390/w17081143

Chicago/Turabian Style

Gallego-Ramírez, Carolina, Yuri García-Zapata, Néstor Aguirre, Edwin Chica, and Ainhoa Rubio-Clemente. 2025. "Acute Toxicity Assessment of Textile Wastewater Treated with Pinus patula Biochar Using Daphnia pulex" Water 17, no. 8: 1143. https://doi.org/10.3390/w17081143

APA Style

Gallego-Ramírez, C., García-Zapata, Y., Aguirre, N., Chica, E., & Rubio-Clemente, A. (2025). Acute Toxicity Assessment of Textile Wastewater Treated with Pinus patula Biochar Using Daphnia pulex. Water, 17(8), 1143. https://doi.org/10.3390/w17081143

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