Reducing the pollution load of tannery wastewater and the atmospheric emission of hydrogen sulfide using modified tannin

The efficient management of industrial wastewater is relevant mainly due to the alert of water shortages worldwide. Around 80% of wastewater is discarded without proper treatment, causing a negative impact on the environment and reducing the availability of uncontaminated water (United Nations Development Program 2019). Industrial wastewater contains pollutants in its composition that can cause environmental and human health issues. Wastewater can also release atmospheric emissions, with hydrogen sulfide (H₂S) being one of the main gaseous contaminants generated in industrial and sanitary wastewater. Hydrogen sulfide is among the most toxic pollutants for humans in the leather industry (Lotti & Bleecker 2015; Lima et al. 2016). H₂S is a colorless gas that produces an unpleasant odor (Aventaggiato et al. 2020), which can be perceived in concentrations between 0.02 and 10 mg/L, while concentrations higher than 1,000 mg/L can lead to death (Mooyaart et al. 2016). Deaths caused by exposure to H₂S usually occur with workers in confined and semi-confined spaces (Godoi et al. 2018; Aventaggiato et al. 2020).

Leather processing is characterized by the presence of wastewater with different physicochemical characteristics due to lots of chemicals used in the process (Hansen et al. 2021a), and characterizing wastewater is relevant to design treatment plants (Lofrano et al. 2013; Sawalha et al. 2019; Hansen et al. 2020). In the beamhouse stage of leather production, when the unhairing and liming processes occur, sodium sulfide is used, which causes the release of H₂S into the atmosphere. The amount of sodium sulfide added in the unhairing and liming process can vary from 1.8 to 2% (Aquim 2009), and the concentration of sulfides in wastewater from this stage can vary from 2,000 to 3,300 mg/L (Sundarapandiyan et al. 2010). Coagulation is one of the oldest methods for treating wastewater, and it continues to be widely used (Ang & Mohammad 2020; Lapointe & Barbeau 2020) due to its low costs (Machado et al. 2020). In the coagulation and flocculation process, inorganic and organic chemicals can be applied as aluminum salts and tannin-based coagulants, respectively (Skoronski et al. 2014; Hameed et al. 2018). Inorganic chemicals (e.g. aluminum sulfate, ferric chloride, and polyaluminum chloride) can cause environmental issues due to the generation of non-biodegradable sludge and human health (Skoronski et al. 2014; Roselet et al. 2016; Kakoi et al. 2017).

Different coagulation techniques have already been used for the treatment of tannery wastewater. Polyaluminum chloride was tested to remove chromium (III) (60–99% removal) and total organic content (20–60% removal) (Tang et al. 2018). Epichlorohydrin-dimethylamine removed around 90% of chemical oxygen demand (COD) and color from wastewater (Niu et al. 2012). Electrocoagulation processes were also effective for the removal of COD (75% removal) (de la Luz-Pedro et al. 2019), and potassium ferrate has been tested to remove COD (77% removal), total organic content (76% removal), suspended solids (97% removal), and color (98% removal). A literature review points out that greater attention should be given to highly effective chemicals that do not harm the environment (Hansen et al. 2021b), with emphasis on the use of vegetable organic chemicals (Skoronski et al. 2014) as modified tannins obtained through the Mannich reaction (Pizzi 2019; Machado et al. 2020). Wastewater treatment using modified tannin-based coagulants is safe and non-toxic and produces biodegradable sludge. Additionally, it eliminates pH adjustment, is non-corrosive, and has a treatment cost similar to inorganic chemicals (Lapointe & Barbeau 2020; Machado et al. 2020). Thus, this study aimed to reduce the pollution load of tannery wastewater and atmospheric emissions of H₂S. The study presents a new proposal for a hybrid treatment to remove contaminants in beamhouse wastewater using a coagulant based on modified tannin, aluminum sulfate, and a combination of both.

Qualitative H₂S emission tests were carried out in a tannery in southern Brazil. Three filters were attached to passive samplers and positioned at the following collection points for 5 min (Pereira et al. 2012):

• Entry of chemicals into the drum (Point A);

• Drum unloading, where the hides are removed (Point B);

• Wastewater treatment plant (Point C).

The raw wastewater was obtained from a tannery with a beamhouse process. Jar tests were performed to evaluate the use of modified tannin (vegetable cationic organic polymer), aluminum sulfate, and the combination of both chemicals. The Compact Jar-test (Milan) was used, with agitation at 180 rpm for 4 min. The definition of the best dosage of coagulant/flocculant and pH for each case was evaluated based on the results for color (SM 2120 B 2017) and turbidity (SM 2130 B 2017). Dosages of 2,000, 2,500, and 3,000 mg/L of modified tannin and aluminum sulfate at pH 6.0 and 9.0 were tested. The dosages were defined based on preliminary tests, with a visual evaluation of the treatment. To assess whether there is synergy between the two coagulants, three different combinations were also tested with 3,000 mg/L of chemicals in the proportions: 80:20, 50:50, and 20:80 (aluminum sulfate: modified tannin). The literature (Costa 2013) indicates that differences in results may occur between treatments with pure coagulants and in combination. This occurs due to the formation of a new chelate compound between the modified tannin and the metal present in aluminum sulfate (Costa 2013).

For the best treatment conditions, the wastewater was analyzed for sulfide (SM 4500-S 2017), COD (SM 5220 C 2017), total nitrogen Kjeldahl (NTK) (SM 4500 Norg 2017), pH (SM 4500 B 2017), color (SM 2120 B 2017), and turbidity (SM 2130 B 2017).

The zeta potential of the raw wastewater was analyzed at the defined pH values for the treatment to verify the stability of the effluents according to the pH. One sample had the original pH of 9.0, and the other sample was adjusted to a pH of 6.0. This pH range is defined in the tannery's Operating Environmental License. The zeta potential was analyzed in the equipment NanoBrook 90PLUSPLAS.

The images of the filters collected after 5 min of exposure to atmospheric air in the tannery are shown in Table 1. The difference in the color of the filters is due to the difference in the concentration of hydrogen sulfide at the collection points.

Table 1

Filters exposed in the tannery to assess H₂S emissions. Please refer to the online version of this paper to see this table in colour: http://dx.doi.org/10.2166/wst.2023.075.

Filter .	Collection point .	Classification .
	Entry of chemicals into the drum (a)	The most concentrated sample
	Drum unloading (b)	The least concentrated sample
	Wastewater treatment plant (c)	The intermediate sample
	Control sample	Zero concentration

Filter .	Collection point .	Classification .
	Entry of chemicals into the drum (a)	The most concentrated sample
	Drum unloading (b)	The least concentrated sample
	Wastewater treatment plant (c)	The intermediate sample
	Control sample	Zero concentration

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The highest concentration of hydrogen sulfide in the atmosphere was identified at the entry point for chemicals into the drum (a). The higher atmospheric emission of H₂S is consistent with the higher presence of sulfides in the lime bath (2,000–3,300 mg/L) (Sundarapandiyan et al. 2010).

The H₂S concentrations in the drum discharge (b) and the wastewater treatment plant (c) were lower than the entry point for chemicals into the drum. This result corroborates the lower concentration in the residual baths (380 mg/L) (Aquim 2009) compared to the original lime bath. This qualitative technique, corroborated by literature data (Rosolina et al. 2016), can identify areas that present a greater or lesser risk to the worker.

The physical–chemical characterization of the raw wastewater collected in the tannery is presented in Table 2.

Comparing the characterization of raw tannery wastewater with data in the literature, some differences are found due to different formulations used in tanneries. The COD of the present study (8,531 mg/L) is higher than that found by Souza (2007) (5,053 mg/L) and Ferrari (2004) (7,250 mg/L) in residual beamhouse baths. The sulfide concentration (282 mg/L) is among the concentrations found by Ferrari (2004) (50 mg/L) and Aquim (2009) (380 mg/L). The presence of total nitrogen (1,137 mg/L) is lower than that found by Souza (2007) (2,642 mg/L) and the turbidity is similar to that found by the author (1,720 NTU).

The results of the zeta potential analysis are shown in Table 3. Wastewater at pH 9.0 was more stable (more difficult to destabilize and promote coagulation) than wastewater at pH 6.0, as the zeta potential was higher at pH 9.0 than at pH 6.0 (Pavanelli 2001).

Results of sulfide removal efficiency (Figure 3(a)) show that the use of modified tannin presented the highest efficiency (62.8%). The positively charged amino groups present in the modified tannin contribute to neutralizing negatively charged particles (Ibrahim et al. 2021) such as sulfides, thus destabilizing them. After no electrical force repels them, the particles can combine to form dense flakes and finally settle (Bolto & Gregory 2007). The pH used in the test with tannin (6.0) helped remove H₂S from the wastewater, as the lower the pH, the higher the concentration of sulfide in the gaseous phase. Sulfur can be present in wastewater in different forms of oxidation. At low pH values, the presence of H₂S (gaseous) predominates, and at high pH values, sulfur is present as HS⁻ and S²⁻ (dissolved) (Chernicharo 2007). However, there are indications that the removal did not happen only because of the difference in pH. The treatment using aluminum sulfate reached pH 7.72 (due to the characteristic of aluminum sulfate lowering the pH of the wastewater); however, the removal efficiency of sulfides was 1.4%. The removal efficiency of sulfides from the test combining chemicals (9ST50:50) was 13.8%, and its pH throughout the test was the closest to the raw wastewater (9.0) and the highest of the three tests. The result of combined chemicals indicates that tannin combined with aluminum sulfate is more efficient than aluminum sulfate alone. The higher sulfide removal with the combined chemicals is related to the availability of positive sites as the modified tannin contains amine groups, and the aluminum sulfate contains a metallic cation.

A coagulation/flocculation technique is considered easy to be applied and cost-effective, compared with other wastewater treatment technologies (Hansen et al. 2021b). However, results found in the literature, applying other techniques, showed higher removals compared to this study. The use of hydrogen peroxide oxidation removed 99% of the H₂S (Hashem et al. 2016), and electrocoagulation achieved 90% pollutant removal efficiency (Sengil et al. 2009). These results indicate that the modified tannin tested in this study can contribute to the removal of sulfides in the primary treatment step; however, complementary treatments are necessary to reach the environmental standards of wastewater disposal as oxidation of sulfide ions by ozone, chlorine, oxygen, hydrogen peroxide, and ammonium persulfate (Bykovsky et al. 2019).

Turbidity (Figure 3(b)) and color (Figure 3(c)) removal efficiencies reached values above 90% and showed similar efficiencies in all tests. When modified tannin is applied, discoloration is usually promoted by adsorption-like coagulation according to the literature (Beltrán-Heredia et al. 2011). The highest efficiency was achieved with the use of aluminum sulfate, followed by the combined chemicals and then the modified tannin. Similar results (above 95% removal efficiency) were found in tannery wastewater at pH 7.0 using modified tannin (Tamogami et al. 2014).

COD removals (Figure 3(d)) ranged from 18 to 30%. Aluminum sulfate was the most efficient, followed by the combination of coagulants and the modified tannin. COD removals of around 60% have already been found in tannery wastewater at pH 7.0 using modified tannin (Tamogami et al. 2014). Total nitrogen removal (Figure 3(e)) had similar removals (around 9%) for both the modified tannin and the combination of coagulants. The test with aluminum sulfate showed 17.6% removal efficiency. The low nitrogen removal efficiencies are consistent with the literature, as nitrogen removal occurs mainly by biological processes (Baur 2012; Pena et al. 2020).

Comparing the performance of the three treatments, aluminum sulfate presented the best results for the removal of turbidity, color, COD, and TKN; however, it showed the worst performance for the removal of sulfides. Modified tannin is not efficient at the original pH of the tannery wastewater (9.0); however, at pH 6.0 (less stable wastewater), tannin is efficient and can replace aluminum sulfate.

Thus, the best option for treatment is the combination of modified tannin with aluminum sulfate, as there is no need for chemicals to decrease the pH of the wastewater, with the potential release of H₂S into the atmosphere (Chernicharo 2007).

The application of modified tannin would require low or no investment, as tanneries often apply the physical–chemical treatment stage with coagulation–flocculation (Hansen et al. 2021b). Thus, only process adjustments, considering the use of the new chemical and its application conditions, would be necessary.

The results found for the treated wastewater were compared with environmental legislation (Table 5) of Brazil (National Environment Council – CONAMA 430, 2011), China (Ministry of Environmental Protection 2013), and India (Ministry of Environment Forest and  Climate Change 2019), countries with important scientific publication in the area of tannery wastewater treatment (Hansen et al. 2021b).

Comparing the coagulation and flocculation treatments with the environmental legislation, it is observed that the treatment is not enough to reach the standards for wastewater disposal in water bodies. However, the use of this technique associated with other biological and advanced treatments can contribute to the removal of turbidity, color, COD, and TKN, in addition to the toxic pollutant H₂S, removing this pollutant from wastewater and reducing its emission to the atmosphere.

This study evaluated the use of modified tannin, aluminum sulfate, and a combination of both for the treatment of tannery wastewater, aiming to reduce its pollution load and hydrogen sulfide emissions. The qualitative test of sulfide in the atmosphere was efficient to identify industrial environments with a higher risk of worker contamination.

Modified tannin showed the highest hydrogen sulfide removal efficiency (62.8%) from the tannery wastewater, and aluminum sulfate showed the highest efficiencies for COD (30.2%), TKN (17.6%), color (97.6%), and turbidity (98.1%). However, the aluminum sulfide treatment presented poor hydrogen sulfide removal (1.4%). Thus, the best treatment indicated in this research was the combination of coagulants, which removed 13.8% of sulfide, 23.3% of COD, 9.1% of TKN, 96.4% of color, and 97.3% of turbidity, as this treatment does not require chemicals to adjust the pH of the wastewater and had superior sulfide removal than aluminum sulfate applied alone.

This treatment technique has a low investment cost and promotes the replacement of 50% of the aluminum sulfate currently used in the tannery by a non-toxic coagulant, which provides a higher removal of sulfides and, consequently, lower atmospheric emission of the gas.

The authors would like to acknowledge the support of Capes, CNPq, and Fapergs.

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

The authors declare there is no conflict.