Adsorption of Boron from a Simulated Soil Solution Using Zn–Al and Mn-Doped Zn–Al Layered Double Hydroxides

Abstract

Three types of layered double hydroxides (LDHs) including Zn–Al and Mn-doped Zn–Al LDHs with Mn 2 and 10 mol % respect to Zn content (Zn–Al, Zn–Mn1–Al and Zn–Mn2–Al, respectively) were prepared. The effect of various parameters such as time, boron concentration, pH, temperature, ionic strength and competing anions were investigated to compare the efficacy of synthesized LDHs for boron adsorption in a simulated soil solution. The results suggest that the adsorption capacity of B is enhanced by incorporation of Mn in the Zn–Al LDH. The highest adsorption was observed for Zn–Mn2–Al probably due to its higher specific surface area as well as due to the ion exchange mechanism in combination with surface adsorption. The kinetics of boron adsorption on the LDHs followed a pseudo-second order model. The isotherm data of Zn–Al was matched with Langmuir model while the sorption data for Zn–Mn1–Al and Zn–Mn2–Al were described by both Langmuir and Freundlich models. Although boron adsorption slightly increased with increasing pH from 5 to 9, it decreases at pH > 9 because of the competition of OH– groups and change in LDH surface charge at pH > pHZPC. The negative values of $$\Delta H_{{\text{r}}}^{{\text{o}}}$$ showed that adsorption of boron was exothermic in nature. The presence of competing anions interfere boron adsorption. Moreover, an increase of electrolyte concentration led to a decrease of boron adsorbed on the LDHs. It seems that Zn–Mn2–Al could be used as an effective sorbent for boron adsorption in soil solution, as it indicated the highest affinity to boron even at high pH and ionic strength conditions.