Magnetic Hybrid Nanosorbents for the Uptake of Paraquat from Water

Although paraquat has been banned in European countries, this herbicide is still used all over the world, thanks to its low-cost, high-efficiency, and fast action. Because paraquat is highly toxic to humans and animals, there is interest in mitigating the consequences of its use, namely by implementing removal procedures capable of curbing its environmental and health risks. This research describes new magnetic nanosorbents composed of magnetite cores functionalized with bio-hybrid siliceous shells, that can be used to uptake paraquat from water using magnetically-assisted procedures. The biopolymers κ-carrageenan and starch were introduced into the siliceous shells, resulting in two hybrid materials, Fe3O4@SiO2/SiCRG and Fe3O4@SiO2/SiStarch, respectively, that exhibit a distinct surface chemistry. The Fe3O4@SiO2/SiCRG biosorbents displayed a superior paraquat removal performance, with a good fitting to the Langmuir and Toth isotherm models. The maximum adsorption capacity of paraquat for Fe3O4@SiO2/SiCRG biosorbents was 257 mg·g−1, which places this sorbent among the best systems for the removal of this herbicide from water. The interesting performance of the κ-carrageenan hybrid, along with its magnetic properties and good regeneration capacity, presents a very efficient way for the remediation of water contaminated with paraquat.

. XRD pattern of magnetite (Fe3O4) nanoparticles.  Figure S4. Control experiments carried out for 24 hours, revealed a negligible loss of paraquat either by glass adsorption or hydrolysis. Figure S5. The effect of contact time on the removal of paraquat from water and adsorption capacity using the -carrageenan hybrid nanoparticles as sorbent.

C. Kinetic modelling
The adsorption kinetics is a fundamental parameter for the application of a material at industrial level. Several models have been established to facilitate the understanding of the adsorption kinetics and the rate-limiting step of a material in relation to an adsorbate. For the -carrageenan particles, three of the most commonly employed models were used to understand the kinetic profile of paraquat to these particles. The equations associated to each model are detailed below [53].

Pseudo 1 st order
The Lagergren's first-order kinetics based on solid capacity is described as follows (S1)[50]: (S1) The integration of the previous equation with the boundary conditions of t = 0, q = 0 and t = t, q = q results (S2): This equation can then be expressed as follows (S3): where, qt and qe (mg.g -1 ) are the adsorption capacity at time t and equilibrium time, respectively; k1 is the equilibrium rate constant of pseudo 1 st order adsorption (min -1 ). Generally, the pseudo-first order kinetics does not result in a good fit in the whole time interval and is usually applicable to the initial part of the kinetic profile.

Pseudo 2 nd order
The pseudo 2 nd order model relies on the sorption capacity of the solid phase, being expressed as follows (S4) [51] (S4) The integration of the previous equation with the boundary conditions of t = 0, q = 0 and at t = t, q = q, result in (S5): The previous equation can then be expressed as follows (S6): where, qt and qe (mg. g -1 ) are the adsorption capacity at time t and equilibrium time, respectively; k2 is the equilibrium rate constant of pseudo 2 nd order adsorption (g.mg -1 .min -1 ). Since the pseudo 2 nd order rate constants can be determined experimentally, it provides a more appropriate representation of the kinetic data in adsorption systems.

Elovich
The Elovich fitting is usually applied when it is assumed that the solid surfaces are energetically heterogeneous [52]. The Elovich model is represented as follows (S7): where,  and  are the Elovich coefficients and define the initial sorption rate (mg. g -1 .min -1 ) and the desorption constant respectively (g.mg -1 ). Figure S6. Modeling of adsorption kinetics of paraquat onto Fe3O4@SiO2/SiCRG particles using pseudo 1 st and 2 nd order kinetic equations and Elovich kinetic model, for an initial paraquat concentration of 30 (top) and 60 g/mL (bottom).

D. Equilibrium Isotherm Modelling
Several equilibrium isotherm models can be used to evaluate the efficacy of the adsorbent. There are many equations that can be used to describe the adsorption equilibrium of an adsorbate on an adsorbent. Under constant temperature conditions, the adsorption isotherms will describe the relationship between the amount of adsorbed adsorbate (qe) and the adsorbate concentration in solution after the equilibrium is reached (Ce) [53,56,57]. In this work, the most common isotherm models were fit to the experimental data for a complete analysis of the k-carrageenan nanoparticles' performance. The adequacy of each model and the respective equation is presented below.

Langmuir isotherm
The Langmuir sorption isotherm applies to systems where the adsorption of the solute occurs as a monolayer on a surface containing a finite number of identical sites. The adsorption must be uniform with no transmigration of adsorbate in the surface plane [22,54,57,58]. The non-linear form of the Langmuir model is given by the following equation (S8): where, qL (mg/g) is the monolayer adsorption capacity per unit of adsorbent and KL (L/mg) is the Langmuir adsorption constant related to the affinity of binding sites.

Freundlich isotherm
The Freundlich isotherm assumes that the adsorption occurs on a heterogeneous surface, resulting in adsorption sites of varying energy. Since this model is based on an exponential equation, it assumes that the concentration of adsorbate on the adsorbent surface increases with the increase of adsorbate concentration. In general terms, this model assumes that the adsorption can occur based on multiple layers [22,54,57,59]. The non-linear form of the Langmuir models is described based on the following equation: where kF (mg (1-1/n) .L (1/n) .g -1 ) is the Freundlich constant and 1/n is the heterogeneity factor which varies between 0 and 1.

Sips or Langmuir-Freundlich isotherm
The Sips model is based on the combination of the Langmuir and Freundlich isotherm models. At low adsorbate concentration, the Sips equation reduces to the Feundlich isotherm, however it predicts a monolayer adsorption at high concentration that is characteristic of the Langmuir isotherm [22,54,57,60]. This model takes the following non-linear form (S10): where Ks is the Sips equilibrium constant (mg.L -1 ) -1/n and Qmax is the Sips maximum adsorption capacity (mg.g -1 ) and nS is Sips model exponent. It is assumed that nS should be between 0 and 1 for integration purposes.
Besides of the usual isotherm models, other fittings were also accomplished for the -carrageenan nanoparticles using alternative models. The equations regarding these fittings are detailed in the table below. Table S1. Other isotherm models that were fit for adsorption of paraquat using the -carrageenan nanoparticles as adsorbent [22,54,57,[61][62][63].

Model Equation (non-linear form) Parameters
Toth 1 Q max is the maximum sorption capacity (mg.g −1 ) K T is the Toth equilibrium isotherm constant (L.mg −1 ) n T is the Toth exponent (dimensionless) Liu 1 Q max is the maximum sorption capacity (mg.g −1 ) K g is the Liu equilibrium isotherm constant (L.mg -1 ) n L is the Liu exponent (dimensionless) (> 0) Hill Q max is the maximum sorption capacity (mg.g −1 ) K H is the Hill equilibrium isotherm constant [(mg.L −1 ) nH ] n H is the Hill exponent (dimensionless) The goodness of the fittings was determined based on the calculation of the correlation coefficient (R 2 ) (S11) and Chi-square test value ( 2 ) (S12), expressed by the following equation respectively: where yi and ŷi are the experimental and model predicted values respectively, is the mean of the experimental data and n is the sample size. The standard free energy change (G o ) was determined using the equation (S13) [66] ∆G° RT ln K (S13) where K is the equilibrium constant for the sorption interaction, T is the temperature in Kelvin and R is the gas constant (8.314 J/(mol.K)). The value of K was calculated using the method suggested by Khan and Singh[65]. This method presupposes that at low concentration the sorption isotherm is a straight line, hence simulating a partitioning process and the equilibrium constant (KKL=qe/Ce) is calculated from the intercept on the ordinate of the plot ln(qe/Ce) against qe (see Figure S7). The value of G o obtained for the adsorption of MTP on Fe3O4@SiO2/SiCRG particles, at 25 °C was -25.19 kJ/mol.