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.

Coagulation-flocculation is widely regarded as a simple and cost-effective method for treating concentrated wastewater of various industries, including pharmaceuticals, petrochemicals, mineral processing, metal production, tanneries, textiles, food, pulp, and paper. This process effectively removes a significant portion of organic and inorganic contaminants, simultaneously decreasing color and turbidity (Meteš et al., 2000). The coagulation-flocculation mechanism involves the destabilization of colloidal particles, forming small aggregates that subsequently grow to form the larger flocs during flocculation. The process efficiency depends on several factors such as: coagulant chemical structure, pH, ionic strength, solid concentration, and particle size distribution within the suspension (Li et al., 2006). For the effective removal of ink nanoparticles, a hybrid treatment approach is often necessary. The combination of this process with adsorption onto zeolite particles has been demonstrated to enhance removal efficiency (Metes et al., 2004). Furthermore, organic compounds can be significantly eliminated through integrated treatment systems, incorporating anaerobic-aerobic degradation and flocculation-precipitation (Wang et al., 2008). Electrocoagulation has been proven to be an effective method for the treatment of wastewater containing printing ink particles, significantly lowering the concentration of colorant compounds (Papadopoulos et al., 2019). Fenton process was combined with coagulation for the synergistic treatment of wastewater discharged from the printing industry (Ma and Xia, 2009; Sayın et al., 2022). Additionally, the integration of physico-chemical treatment with nanofiltration has been explored as a viable strategy to facilitate the water recovery process (Bes-Pia et al., 2003). Among advanced remediation approaches, the hydrodynamic cavitation coupled with hydrogen peroxide indicated the high effectiveness in removing ink particles and reducing chemical oxygen demand (COD) (Zampeta et al., 2021, 2022a).Coagulants play a crucial role in the recovery of industrial wastewater contaminated with printing inks. Pre-hydrolyzed coagulants, such as polyaluminum chloride (PAC), polyaluminum ferric chloride (PAFCl), polyferrous sulfate (PFS), and polyferric chloride (PFCl), have been widely employed in wastewater treatment (Nandy et al., 2003; Verma et al., 2012). The coagulation-flocculation in the presence of PAC is considered to be a potentially feasible process for the removal of turbidity, metals, and organic materials as compared to alum. PAC is a mixture of Al3+ and polymeric aluminum cations, including Al2(OH)24+, Al8(OH)204+, AlO4Al12(OH)24(H2O)127+ (Yang et al., 2011), in which the last compound is the most effective species in the coagulation-flocculation (Gao et al., 2005). AlO4Al12(OH)24(H2O)127+ is a pre-hydrolyzed coagulant with a high positive charge as compared to Al3+ (Hu et al., 2006). Consequently, the use of PAC as coagulant results in better performance in the treatment of effluents (Wang et al., 2015). On the other hand, the toxicity of aluminum-based coagulants is attributed to the concentration of Al3+ which are more available to the organisms than polymeric aluminum compounds (Mortula et al., 2013). The residual Al3+ in the treated water was also found to be in a lower concentration when PAC is employed compared to other aluminum-based coagulants (Kimura et al., 2013). While PAC has been identified as the most efficient coagulant, ferric chloride shows limited removal efficiency in the recovery of wastewater contaminated with ink particles (Nandy et al., 2004). In contrast, coagulation using PAC and ferrous sulfate enhances the Fenton process, improving both color and COD removal efficiency (Ma and Xia, 2009). The efficiency of COD removal depends on several factors, including pH, coagulant dosage, mixing time, and speed (Fendri et al., 2013; Shaheed et al., 2020). The addition of PAC to wastewater 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.