Extensive urbanization and industrialization have caused a six-fold increase in the demand for unpolluted water (Nzediegwu et al., 2019), resulting in a global scarcity of potable water. Anthropogenic activities have degraded freshwater quality worldwide by introducing industrial and agricultural contaminants (Mohan et al., 2014). This issue is particularly severe in countries like Pakistan, an agricultural state in a semi-arid region with limited freshwater resources. As a large volume of potable water is consumed for irrigation of agricultural lands (FAO. AQUASTAT, 2016; UNESCO, 2016), farmers are dependent on the consumption of sewage water as an alternative to freshwater.

The use of wastewater for irrigation has been proposed and encouraged by many researchers to tackle the problem of freshwater scarcity (Rusan et al., 2007; Dhiman et al., 2020). However, in developing countries, the release of untreated wastewater into freshwater resources is a common practice, causing the contamination of arable land when irrigated with untreated wastewater (Qadir et al., 2010). Wastewater comprises nutrients; surfactants; potentially toxic elements (PTEs) like cadmium (Cd), chromium (Cr), nickel (Ni), and lead (Pb) (Khan et al., 2008); and organic pollutants (Khalid et al., 2018). Therefore, the increased wastewater generation caused by extensive urbanization and industrialization demands the safe disposal of wastewater.

PTEs are considered environmentally toxic pollutants and substances that pose health risks, and their accumulation in animals and humans has been aggravated by anthropogenic activities (Sabir et al., 2022a). In developing countries like Pakistan, due to lack of awareness and resources, poor management of industrial waste contributes to the spread of PTEs in the environment. This accumulation of PTEs in the soil system is a matter of concern for growing industries in developing countries. The wastewater contains trace metals from domestic, commercial, surface runoff, and industrial origins (Fuerhacker et al., 2010), and their presence in the soil induces toxicity by ionic imbalance, leading to declined growth in plants.

Chromium toxicity in plants resulted in reduced germination, growth, and disturbed photosynthesis, nutrient and water uptake, and enzymatic activity. This triggers the production of reactive oxygen species (ROS) to oxidize biomolecules causing plant death (Ekere et al., 2020). High toxicity, persistent nature, increased bioavailability in open environments, and resulting bioaccumulation and biomagnification are the important factors causing health risks in plants, animals, and humans (Naveed et al., 2021). Various approaches (Jiang et al., 2022) such as chemical methods (Allegre et al., 2004), bioremediation (Naseem et al., 2023), physical treatment (Abbas et al., 2015), and phytoremediation (Naveed et al., 2021) have been reported for remediation of PTE-contaminated soils (Jan et al., 2021). However, adsorption by biochar (BC) and polymers is the most economical, practical, efficient, effective, and eco-friendly approach among all the available techniques for immobilization of PTEs.

Super adsorbent polymers, sometimes called hydrogels, are highly hydrophilic networks of loosely crosslinked polymer chains that can absorb and retain up to hundreds of times their weight of water or aqueous solutions (Dhiman et al., 2020) and slow water release, which could improve the growth of the plant in water stress conditions (Beckett and Augarde, 2013), supply the cation, and decrease the availability of some toxic elements. Superabsorbent polymers (SAPs), also identified as soil polymers or macromolecular polymers, are proficient in repeatedly absorbing, retaining, and releasing extremely large amounts of water, relative to their own weight. SAPs can absorb more than a thousand times their original weight in water, and they can retain liquids even under pressure (Liu et al., 2009). These polymers are applied in agriculture to improve the soil’s physical properties like water- and nutrient-holding potential and promote crop growth in arid and semi-arid areas. Recently, the most commoditized polymers are mainly polypropylic acid (PAA) or polyacrylamide (PAM) (Islam et al., 2011; Li et al., 2022).

Acrylamide is a white, colorless, chemical compound that is soluble in water, ethanol, ether, and chloroform and is produced synthetically. The large surface area and presence of various functional groups make usage of polymers ideal in various fields like agriculture, horticulture, biomedicine, bioengineering, food storage, and treatment of wastewater (Li et al., 2022). Modified-polyacrylamide hydrogels are used commercially for the purification of wastewater and metal extraction (Dhiman et al., 2015). Due to their hydrophilic nature and carboxylic functional groups, polymers can tightly bind to PTE, thereby minimizing the uptake of heavy metals by plants (Huttermann et al., 2009). Synthesized polymers have been used extensively with adsorbents for heavy metal removal since the surface properties of the adsorbents can be modified by enhancing available functional groups to improve their adsorption ability for pollutants (Wang et al., 2015). One way to decrease PTE availability to plants is by increasing binding sites for PTEs in soil through amendment application.

Biochar is the end product of pyrolysis, carbonization, and gasification of plant and animal-based materials (ASABE, 2011) and contains a high surface area, numerous pores, various oxygen-containing functional groups, and high cation exchange capacity with alkaline pH (Lehmann, 2007; Al-Wabel et al., 2017). BC application may improve crop yield by cultivating polluted land. A more recent study (Nie et al., 2018) showed that sugarcane bagasse-derived BC significantly reduced Cd, Cu, and Pb uptake by Pak Choi (Brassica chinensis L.) plants grown in wastewater-polluted soil, significantly increasing the yield. A pot experiment study with tobacco stem-derived BC showed that the application of BC led to immobilization of Cr, Cu, and Pb in soil (Zhang et al., 2019). Additionally, BC can serve as a low-cost adsorbent for heavy metal removal in wastewater treatment plants (Inyang et al., 2016).

Generally, soils are good accumulators of PTEs, and application of BC in the topsoil leads to further immobilization of PTE. Due to its diverse surface characteristics, BC is known to be effective in the adsorption of PTE (Lehmann, 2007). Modification of BC with foreign materials such as silica, zeolites, polymers, and nutrient enrichment improves its physiochemical properties and ameliorates its efficiency and environmental influence (Ok et al., 2015; Rafique et al., 2022). The use of BC amendments along with synthetic (acrylamide, polyurethane, polyvinyl, and resins) and natural polymer derivatives of algal polysaccharides has shown a promising influence on the immobilization of chromium in soil (de-Bashan and Bashan, 2010).

Considering the interaction of wastewater irrigation, soil PTE, and polymers and BC, it is suggested that employing cost-effective techniques, such as incorporating polymers and BC into the soil, could be safe for using sewage water in arable soils In previous studies, the use of BC with other organic amendments like compost has been reported (Coelho et al., 2018; Naveed et al., 2021). Polymers along with BC induce a large number of binding sites to hold cationic contaminants and sequester carbon from the environment (Ekebafe et al., 2012). Polymers also help reduce BC pH and improve its performance in alkaline soils as well.

The simultaneous use of polymers and BC emerges as an effective approach to enhance the growth and yield of plants facing heavy metal stress. Application of polymers in soil will increase immobilization of PTE and improve soil properties like water- and nutrient-holding capacity, which would be beneficial for plant growth. The use of BC will help in increased immobilization of PTE in soil, providing essential nutrients. Moreover, the combined application of BC and polymer has not been reported previously. Therefore, we hypothesized that the co-application of BC and polymer could be a better strategy to mitigate risks of PTEs, while enhancing soil properties like water-holding capacity, nutrient availability and organic matter, and improving the growth, physiology, and yield of plants. The objectives of this experiment were to evaluate the impact of polymer–BC and their combined application on growth, physiology, water relations, yield of pea plants grown under different levels of sewage water along with the phytoremediation potential, and associated health risks of pea plants grown with sewage water containing Cd and Cr PTE.

Contamination of soil with potentially toxic elements has been responsible for an indisputable impact on the plants inhabiting polluted soil. However, the impact of toxicity caused by these pollutants on plants amended with BC and polymer in metal-contaminated soils remains unclear and is, therefore, the subject of the present research.

The soil without amendments and irrigated with different levels of sewage water showed reduced shoot and root length, shoot fresh and dry weight, and root fresh and dry weight in pea plants (Figure 1) due to the presence of heavy metals like cadmium (Cd) and chromium (Cr). Various researchers reported reduced growth parameters under PTE stress (Gill and Tuteja, 2010; Maqbool et al., 2018; Naveed et al., 2021; Sabir et al., 2022a). Cr toxicity inhibits cell division and elongation in plant root cells (Adrees et al., 2015), thus reducing the root surface area for enhanced water and nutrient uptake from the soil (Medda and Mondal, 2017; Ahmad et al., 2020) and ultimately growth and biomass production. Our findings are supporting by those of studies by Jun et al. (2009) and Naveed et al. (2021) that Cr-contaminated soil reduced shoot length, root growth, and biomass production in cereals, vegetables, and forages.

The structural and functional properties of BC and polymers are well known to stimulate the growth of plants under metal stress. The high cation exchange capacity, water-holding capacity, and gradual release of nutrients by the polymer and BC facilitated increased growth in pea plants. Results showed that the application of polymer and BC significantly (p< 0.05) improved the growth attributes of pea plants (Figure 1) in both sewage water- and tap + sewage water-contaminated soils. The polymer has a large surface area with various functional properties like high nutrient and water-holding capacity with slow release. These characteristics help in the provision of nutrients and water upon need (Zhang et al., 2013). BC is a porous material produced from the pyrolysis of biomass and hence restores the macro and micronutrients in BC. Rafique et al. (2020) reported that polymer-treated BC increased the growth of plants under Cr toxicity. Our finding of increased biomass and length of pea plants with the application of polymer and BC under sewage water irrigation aligns with those of the previous studies (Wang and Xu, 2013; Rafique et al., 2021). Therefore, the combined application of polymer and BC showed significant growth of peas under irrigation with tap and sewage-contaminated water.

The application of metal-contaminated sewage water reduced physiological attributes such as chlorophyll contents and relative water contents, while increasing electrolyte leakage and osmotic potential (Figure 2). The reduced physiological attributes could be due to continuous irrigation using low water quality, which may lead to an accumulation of PTE in soil and plants (El-Hassanin et al., 2020), which disrupts plant physiology due to production of ROS that oxidizes biomolecules under heavy metal stress (Sabir et al., 2022b) and threatens soil biology, like plant growth promoting bacteria, soil enzymes, organic matter contents, and nutrients and water contents in soil (Ahmad et al., 2013; Cheng et al., 2020). These factors have a great impact on various plant physiological activities. Previous studies also confirm this finding (Ali et al., 2018; Saffan et al., 2022).

The presence of nutrients and organic matter in sewage water improved physical and chemical properties along with the increased nutrient status of soil, therefore triggering improved physiological activities of pea plants when polymer and BC were used. The immobilized PTE by polymer and BC showed enhanced chlorophyll contents and relative water contents, with reduced electrolyte leakage and osmotic potential in pea plants under sewage water irrigation. This finding is aligned with previous findings (Arshad et al., 2017; Waheed et al., 2019) that wastewater irrigation significantly augmented organic matter contents, which comprises numerous types of organic compounds and inducing fertility of soil with immobilized PTE, thus inducing a positive response on overall plant physiology (Figure 7). Thus, with notable nutrients and organic matter content, the usage of sewage water is encouraged by researchers (Akbar et al., 2021; Naseem et al., 2022).

The use of sewage water for irrigation of pea plants reduces the yield of pea plants (Figure 3). The presence of PTE and salts among various pollutants significantly affects the development of pea pods and pea grains when sewage water was applied. The pollutants in sewage water reduced the nutrient uptake, which caused an imbalanced uptake of nutrients. This could lead to poor development of plant tissues, and hence yield was compromised. However, with the application of BC and polymer, pea plants showed increased yield when sewage water was applied with tap water (Figure 3). A significant increase in yield was observed with the combined use of polymer and BC when tap water was mixed with sewage water for irrigation. Functional groups like hydroxyl and carboxylic functional groups facilitate metal binding (Mehmood et al., 2021; Murtaza et al., 2021).

Moreover, the high surface area of both BC and polymers provides more adsorption sites for heavy metal binding, whereas increased adsorption of PTE by polymer due to electrostatic interactions (Inyang et al., 2010; Nigussie et al., 2012) resulted in reduced metal content in soil. BC and polymer ensured the uptake of nutrients in plants essential for the proper functioning of plants, which resulted in improved development of pea plants and hence enhanced the yield of pea plants (Jun et al., 2009). Application of BC affected soil properties like pH, cation exchange capacity (CEC), soil organic matter, and nutrient availability and ultimately increased soil fertility and plant growth (Agegnehu et al., 2015).

Reduced Cr and Cd uptake in root, shoot, and grains of pea plants was found with application of amendment polymer and BC when sewage water was applied for irrigation. These amendments helped in the mitigation of the toxicity of sewage water and hence immobilized the metals Cr (Figure 4) and Cd (Figure 5). The presence of oxygenated and hydrogenated functional groups on the BC surface might have formed complexation with CrVI and reduced its availability (Beesley et al., 2014; Niamat et al., 2019). Moreover, it is reported that Cr VI is converted to Cr III by BC, hence reducing the toxicity (Shahid et al., 2017; Wang et al., 2019).

It was found that BC immobilized soil Cr, thus reducing its bioavailability, which is consistent with the findings of Islam et al. (2021). Moreover, lower plant availability of CrVI, as affected by applied BC, could be due to the presence of more active and binding sites on the BC’s surface, which strengthens its ability in fixing PTE (Ekebafe et al., 2012; Wang et al., 2015). Moreover, BC can reduce PTE bioavailability by influencing soil pH, water contents, adsorption, and changing HM redox state (Ditta et al., 2016; Rizwan et al., 2016; Cheng et al., 2020).

Polymers have remedial potential due to the presence of ionic functional groups, which help in their bonding with PTE (Dhiman et al., 2015). Polymers have significant potential to adsorb water in comparison to other conventional adsorbents. Acrylamide base polymer showed effective remediation (>75%) of PTE cadmium, cobalt, copper, and nickel from water (Moreno-Sader et al., 2019). The presence of highly dense metal chelating groups in some polymers made them perfect for the immobilization of PTE in soil, and hence reducing their bioavailability. The reduced uptake of Cd and Cr in pea plants in our study is supported by these findings. Moreover, all treatments receiving SW alone as well as TW + SW showed values of EF and BAC less than 1, for both Cd (Figures 4E, F) and Cr (Figures 5E, F) in pea plants. The health risk assessment parameters of DIM, HQ, and LCR for Cd (Table 1) and for Cr (Table 2) showed that consumption of pea plants irrigated with TW + SW in the presence of polymers and BC is safe.

We found that the application of sewage water negatively influenced growth, physiological parameters, and yield of the pea plant. Combined application of BC and polymer significantly alleviated PTE (i.e., Cd and Cr) stress and enhanced the growth and yield of pea plants. The combined impact of BC and polymer enhanced the immobilization of PTEs like Cd and Cr in soil, which ultimately resulted in reduced bioaccumulation of PTE in pea plants irrigated with sewage water. Moreover, the usage of same amendments significantly reduced the health risks associated with the consumption of pea plants under Cd and Cr stress, as indicated by the lowest values of DIM, HQ, and LCR, when sewage water was used for irrigation. In conclusion, the application of PAM-BC association could serve as an effective approach in mitigating the toxicity of PTE in pea plants irrigated with TW + SW by immobilization of PTE and their associated health risks. Application of BC and PAM could be used as a potential approach to remediate sewage water toxicity and improve growth and yield of plants. The impact of BC and PAM, both as individual and combined application, on soil quality should be studied at the molecular level. More studies should be conducted to find the optimized dose rate to promote soil microbial activity, plant growth, and immobilization of PTE with use of tap water + sewage water for irrigation.