One-Step Room-Temperature Synthesis of Bimetallic Nanoscale Zero-Valent FeCo by Hydrazine Reduction: Effect of Metal Salts and Application in Contaminated Water Treatment

The effect of initial salt composition on the formation of zero-valent bimetallic FeCo was investigated in this work. Pure crystalline zero-valent FeCo nanoparticles (NPs) were obtained using either chloride or nitrate salts of both metals. Smaller NPs can be obtained using nitrate salts. Comparing the features of the FeCo prepared at room temperature and the solvothermal method revealed that both materials are almost identical. However, the room-temperature method is simpler, quicker, and saves energy. Energy-dispersive X-ray (EDX) analysis of the FeCo NPs prepared using nitrate salts at room temperature demonstrated the absence of oxygen and the presence and uniform distribution of Fe and Co within the structure with the atomic ratio very close to the initially planned one. The particles were sphere-like with a mean particle size of 7 nm, saturation magnetization of 173.32 emu/g, and surface area of 30 m2/g. The removal of Cu2+ and reactive blue 5 (RB5) by FeCo in a single-component system was conformed to the pseudo-first-order and pseudo-second-order models, respectively. The isotherm study confirmed the ability of FeCo for the simultaneous removal of Cu2+ and RB5 with more selectivity toward Cu2+. The RB5 has a synergistic effect on Cu2+ removal, while Cu2+ has an antagonistic effect on RB5 removal.


Adsorption data analysis
The removal performance of the alloy at any time t was determined by either the amount of Cu 2+ and RB5 adsorbed onto one gram of the alloy (qt, Eq S1) or the removal percentage (R%, Eq. S2).
where Ci and Ct (mg/L) are the concentration of Cu 2+ or RB5 at time 0 and t, respectively, V (L) is the volume of Cu 2+ or RB5 solution, and m (g) is the used mass of the alloy.

Pseudo-first-order model
The rate constant of adsorption is determined from the pseudo-first-order equation given by Lagergren and Svenska 1 as follow: where qe (mg/g) is the adsorption capacity at equilibrium, qt (mg/g) is the amount of solute adsorbed on the adsorbent at time t, k1 (min −1 ). is the pseudo-first order rate constant and t (min) the time.

Pseudo-second-order model
The pseudo-second-order equation 2 based on equilibrium adsorption is expressed as: where k2 (g/mg min) is the rate constant of the second-order adsorption.

Elovich model
This model is one of the equations that best describes the activated chemical adsorption. It is suitable in systems that have heterogeneous adsorbing surfaces 3 .
where qt (mg/g) is the amount of adsorption at time t = t, β is Elovich constant (g/mg), α is initial adsorption rate (mg/(g min)) Adsorption isotherm models

Freundlich model
This empirical model can be used to describe non-ideal distribution of heat of adsorption and affinities on a heterogeneous surface, it is not restricted to the formation of monolayer 4 . The nonlinear form can be presented by Eq. S7.

Langmuir model
Adsorption isotherm of single adsorptive was analyzed with the Langmuir model, which is used to describe a monolayer adsorption onto the surface of an adsorbent with finite number of identical adsorption sites 5 , it can be written in non-linear form as: where kL (L/mg) is the Langmuir constant and qL (mg/g) is the monolayer adsorption capacity of the adsorbent.

Temkin model
Temkin model assumes that adsorption is a multi-layer process, and neglects the extremely low and high concentrations 6 . Eq. S8 gives the non-linear form of this model.
where AT (L/g) is Temkin equilibrium binding constant, bT (J/mol) is Temkin constants, R is the universal gas constant (8.314 J/mol K), and T is the absolute temperature (K).

Error analysis
The error functions have been designed to evaluate the reliability of the models and identify the best model that describe and foresee the adsorption process 7 . Three error functions were applied S4 in this study, specifically, coefficient of determination (R 2 , Eq. S9), nonlinear chi-square (χ 2 , Eq. S10), and root mean square error (RMSE, Eq. S11).
where qe,exp is the experimental adsorption capacity at equilibrium, qe,cal is the calculated adsorption capacity at equilibrium, N is the number of experimental data, ̅ , is the average experimental capacity at equilibrium, and M is the number of variables of the model. Figure S1.