Application of QSAR for investigation on coagulation mechanisms of textile wastewater

Coagulation is an effective preliminary treatment process for textile wastewater. In order to evaluate the effectiveness of the coagulation process, we performed quantitative structure activity relationship (QSAR) analyses with total organic carbon (TOC) removal rates (R_exp) as an index by three different coagulants (AlCl₃, FeCl₃, and MgCl₂). The experimental results showed that the average R_exp of the three coagulants was 39.12% ± 2.60%, 51.60% ± 2.88%, and 49.95% ± 3.17%. Subsequently, 42 molecular descriptors of dye molecules were calculated by quantitative calculation softwares Gaussian 09, Material Studio 7.0, and Multiwfn 3.7, and then QSAR models were constructed by a multiple linear regression (MLR) method for the three coagulation systems. The developed QSAR models demonstrated excellent stability, robustness, and predictability with values of R² = 0.7677, 0.8015, and 0.7035, Q²_INT = 0.6067, 0.7026, and 0.5898, Q²_EXT = 0.5505, 0.5076, and 0.5697, respectively. Based on the analysis of quantum parameters, the coagulation mechanism for AlCl₃, FeCl₃ is primarily electrostatic adsorption as well as hydrogen bonding, while MgCl₂ coagulates dyes mainly by electrostatic adsorption. This study provides an assessment of the removal rules and a feasible method for predicting dye removal rates in AlCl₃, FeCl₃, and MgCl₂ coagulation process.