Intrinsic Dependence of Groundwater Cation Hydraulic and Concentration Features on Negatively Charged Thin Composite Nanofiltration Membrane Rejection and Permeation Behavior.

Miroslav Kukučka,Nikoleta Kukučka Stojanović

doi:10.3390/membranes12010079

Abstract

Commercial nanofiltration membranes of different molecular weight cut-offs were tested on a pilot plant for the exploration of permeation nature of Ca, Mg, Mn, Fe, Na and ammonium ions. Correlation of transmembrane pressure and rejection quotient versus volumetric flux efficiency on nanofiltration membrane rejection and permeability behavior toward hydrated divalent and monovalent ions separation from the natural groundwater was observed. Membrane ion rejection affinity (MIRA) dimension was established as normalized TMP with regard to permeate solute moiety representing pressure value necessary for solute rejection change of 1%. Ion rejection coefficient (IRC) was introduced to evaluate the membrane rejection capability, and to indicate the prevailed nanofiltration partitioning mechanism near the membrane surface. Positive values of the IRC indicated satisfactory rejection efficiency of the membrane process and its negative values ensigned very low rejection affinity and high permeability of the membranes for the individual solutes. The TMP quotient and the efficiency of rejection for individual cations showed upward and downward trends along with flux utilization increase. Nanofiltration process was observed as an equilibrium. The higher the Gibbs free energy was, cation rejection was more exothermic and valuably enlarged. Low Gibbs free energy values circumferentially closer to endothermic zone indicated expressed ions permeation.

Highlights

Dominant constituents of groundwater include inorganic salts represented by hydrated cations and different anions as counter ions
The authors of this paper present a different approach to better understanding the permeation characteristics of the commercial polymeric asymmetric thin-film composite nanofiltration membranes when exposed to natural groundwater
The obtained results mean that Ion rejection coefficient (IRC) and its suggested the dispersion of a k (SDk) values indicate the nature of the ion separation mechanism at the membrane surface

Summary

Introduction

Dominant constituents of groundwater include inorganic salts represented by hydrated cations and different anions as counter ions. Representing anions are bicarbonate, chloride, sulfate, and phosphate. Besides those mentioned, many other chemical elements as ions and dissolved organic matter are present in natural underground water solution, known as groundwater. Pressure-driven process with nanofiltration (NF) membranes is one of the most utilized ways for successful groundwater softening and partial demineralization [3], as well as drinking water production [4]. Reverse osmosis is a commonly used pressure-driven process for groundwater treatment. Comparison of nanofiltration and the reverse osmosis process was conducted recently by Cai et al, where Sr2+ and natural organic matter rich water was treated with applied pressure and pH. It was reported that overall better rejection was obtained using reverse osmosis membranes, probably due to the nanofiltration membranes’ large pore size [5]. As reported by Riuz-García et al, if requirements for total dissolved solids or some specific harmful ions in the permeate are not as high, in

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