Identification of proper inorganic coagulant for treatment of industrial wastewater contaminated with printing inks through coagulation-flocculation in absence and presence of polyacrylamide: Nanoparticle emissions and mechanical stability.

The effluents discharged from cardboard packaging industries are often contaminated with printing inks. In this study, the thick wastewater, 34.7 g L, was treated through the coagulation-flocculation process to separate ink particles. Inorganic coagulants, magnesium chloride (MgCl), ferric chloride (FeCl), 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, MgCl 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 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, MgCl 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.