Abstract
Effluent reuse is a common practice for sustainable industrial water use. Salt removal is usually carried out by a combination of membrane processes with a final reverse osmosis (RO). However, the presence of silica limits the RO efficiency due to its high scaling potential and the difficulty of cleaning the fouled membranes. Silica adsorption has many advantages compared to coagulation and precipitation at high pHs: pH adjustment is not necessary, the conductivity of treated waters is not increased, and there is no sludge generation. Therefore, this study investigates the feasibility of using pseudoboehmite and its calcination product (γ-Al2O3) for silica adsorption from a paper mill effluent. The effect of sorbent dosage, pH, and temperature, including both equilibrium and kinetics studies, were studied. γ-Al2O3 was clearly more efficient than pseudoboehmite, with optimal dosages around 2.5–5 g/L vs. 7.5–15 g/L. The optimum pH is around 8.5–10, which fits well with the initial pH of the effluent. The kinetics of silica adsorption is fast, especially at high dosages and temperatures: 80–90% of the removable silica is removed in 1 h. At these conditions, silica removal is around 75–85% (<50 mg/L SiO2 in the treated water).
Highlights
Chemical and Materials Engineering, Faculty of Chemistry, Complutense University of Madrid, Citation: Miranda, R.; Latour, I.; Abstract: Effluent reuse is a common practice for sustainable industrial water use
Membrane treatments have gained considerable acceptance in urban and industrial water reuse as they are very efficient in producing high quality water, while they are easy to operate and can be combined with other treatment processes [1,2]
The equilibrium silica uptake loading capacity was obtained by Equation (4), where Ce is the equilibrium concentration of silica in solution. These data were analyzed by Langmuir and Freundlich isotherms, while thermodynamic parameters such as standard free energy, enthalpy, and entropy changes were calculated from the temperature dependence of equilibrium constants using the methodology described in the Supplementary Material
Summary
Industrial water reuse is an important issue because of the lack of fresh water, stringent environmental legislation, and high costs of water treatments. Of the few studies carried out for silica removal by adsorption, all agree that activated alumina is highly selective for silica removal [26,33,34], and this is an important advantage when complex industrial wastewaters are treated. There are insufficient fundamental studies to ensure the success of its application at the industrial scale, and the objective of the study described here is to study silica removal by adsorption onto activated alumina (γ-Al2 O3 ) and its precursor (pseudoboehmite) at various sorbent dosages, pHs, and temperatures. Deinking paper mills are characterized by having a high concentration of silica in process waters and in the effluent (50–250 mg/L) [24,25,37,38] resulting from the sodium silicate added to achieve the optical properties required in the final product. Without severe scaling problems, making this kind of effluent reuse scheme not feasible [14]
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