2), ferric chloride (FeCl3), and polyaluminum chloride (PAC), were applied both individually and in combination with polyacrylamide (PAM) as a coagulant aid. The mixture design methodology (MDM) was used to evaluate the effect of coagulant combinations on water recovery efficiency and settling time. The obtained experimental results indicated that the combination of coagulants negatively impacted the treatment efficiency. While the hybrid coagulant, containing PAC and PAM, proved to be effective in coagulation-flocculation, MgCl2 was found to be the most efficient additive in the absence of PAM. The removal efficiency exceeding than 93 %, could be attained for chemical oxygen demand (COD), turbidity, and colorants at a coagulant dosage of 10.0 g L−1 in which the pH level plays a critical role. A shift in zeta potential from +0.34 to −4.5 mV significantly reduced the settling time to less than 5 min by adding the hybrid coagulant. However, the formation of mechanically unstable flocs resulted in the release of ink nanoparticles, 59–80 nm, into the treated water. By considering this limitation, MgCl2 is recommended for the coagulation-flocculation of ink particles to produce flocs with enhanced mechanical stability and resistance to shear-induced breakup. The proposed approach offers a simple, cost-effective, and eco-friendly route for treating industrial wastewater contaminated with the ink nanoparticles.

The rapid expansion of cardboard packaging industries has resulted in a substantial increase in printing ink consumption (Zięba-Palus and Trzcińska, 2011), leading to highly colored wastewater contaminated with hazardous constituents such as pigments, dyes, resins, binders, and solvents. The discharge of these effluents without proper treatment threatens aquatic ecosystems, agricultural systems, and public health due to contamination of soil, surface water, and groundwater (Ding et al., 2024). Therefore, the effective treatment of wastewater contaminated with printing inks is a crucial issue from the environmental viewpoint. Although conventional remediation techniques, including biological treatment (Zhang et al., 2003), chemical oxidation (Zhang et al., 2021), adsorption (Noonpui et al., 2010), electrochemical method (Ramos et al., 2019), membrane filtration (Zhang and Liu, 2003), photocatalytic degradation (Vitale et al., 2023), and electrocoagulation (Zampeta et al., 2022b), have been applied, the efficacy remains limited, often failing to meet stringent effluent quality standards.

ter treatment processes has been shown to significantly improve the overall treatment performance, particularly in the removal of color and organic matter. The effectiveness of PAC can be attributed to its unique chemical properties, which facilitate the aggregation of suspended particles and enhance the settling process. Moreover, the use of PAC in combination with other treatment methods, such as biological treatment or advanced oxidation processes, can lead to even greater improvements in effluent quality. As industries continue to seek more sustainable and efficient wastewater treatment solutions, the application of coagulants like PAC will remain a critical area of research and development.ter leads to complete color removal, though this process requires an extended settling period. Magnesium chloride provides shorter settling times compared to alum and PAC (Tan et al., 2000). Additionally, polymerized aluminum magnesium chloride (PAMC) represented better performance than PAC in removing turbidity, colorants, and COD from printing wastewater (Yang et al., 2024). Magnetic flocculants, synthesized via solid-state reactions, have also been successfully applied in treating wastewater, containing inks (Ding et al., 2021). Although polyferric chloride is not always effective for COD reduction, poly-silicate-aluminum-ferric chloride (PSAFC) exhibits superior organic removal capability compared to PAC (Yuan et al., 2006).

Polyacrylamide (PAM), a water-soluble synthetic polyelectrolyte, exhibits a high affinity for bonding with suspended particles. Due to the non-ionic, anionic, and cationic forms of PAM, this material significantly improves the flocculation potential in wastewater treatment (Harif et al., 2023). Notably, anionic PAM reduces settling time when it is used as a coagulant aid (Zampeta et al., 2022c). The combination of PAC and cationic PAM showed an effective role in removing colorants from the wastewater of cardboard industry (Nath and Pande, 2020). The primary use of PAM in wastewater treatment is to bridge the coagulated particles in the presence of inorganic coagulants such as PAC (Nan et al., 2016). Polyacrylamide with opposite charge respect to the suspended particles are strongly adsorbed to reduce the electrical repulsion (Zhu et al., 2018; Habibi et al., 2024). For a non-ionic polyacrylamide, or PAM with the same charge of particles, adsorption occurs through the hydrogen bonds of individual polymer chains, forming molecular bridges between the adjoining particles (Peiris et al., 2010). The exposed amide groups of non-ionic PAM provide superb conditions for interaction with particles. Alternative approaches, such as the combination of chitosan and tannin revealed significant efficacy in the ink removal from the effluents (Roussy et al., 2005). Furthermore, the flocculants derived from wood pulp, demonstrate superior performance in the coagulation-flocculation process compared to PAC combined with PAM (Guo et al., 2021). While inorganic coagulants are frequently used due to low cost and easy application, organic polymeric flocculants have gained increasing interest because of exceptional treatment efficiency. As a result, the biodegradable biopolymers have emerged as a sustainable alternative (Lee et al., 2014). The performance of bio-flocculants in removing organic pollutants from the wastewater depends on mechanisms such as adsorption, charge neutralization, and chemical reactions (Li et al., 2020). Inorganic-organic hybrid coagulants also demonstrate high turbidity removal efficiency (Abujazar et al., 2022). However, the sludge generated from conventional wastewater treatment is often toxic and non-biodegradable, posing significant environmental risks. In comparison, plant-based coagulants offer a sustainable alternative due to the biodegradability, non-toxicity, and cost-effectiveness (Owodunni and Ismail, 2021). Although several studies reported the treatment of wastewater contaminated with printing inks through the coagulation–flocculation process, the objective of current study is the identification of a proper hybrid coagulant, containing magnesium chloride (MgCl2), ferric chloride (FeCl3), and polyaluminum chloride (PAC), to maximize the water recovery efficiency and minimize the settling time. The primary focus of study is to enhance the mechanical stability of flocs under the vigorous shear forces to prevent the release of ink nanoparticles which departs from the previous research. These inorganic materials were selected to achieve an effective hybrid coagulant in the absence and presence of PAM, which is crucial in the control of mechanical stability. Moreover, to systematically understand the interactions among coagulants for facilitating the treatment of industrial wastewater discharged from printing unit was investigated via mixture design methodology (MDM) and response surface method (RSM). The findings offer the valuable insights in the development of a facile, cost effective, and eco-friendly method to achieve maximal removal of COD, turbidity, and colorants by determining the proper operating conditions, including coagulant dosage and pH level.