Intercalation of carbon quantum dots into the selective layer of water softening membranes for improved performance and antifouling properties

Abstract

Nanofiltration contributes to water softening by the exclusion of multi-valent hardness ions, through size exclusion mechanisms. Hardness reduction can be enhanced by the addition of positive charges to the selective layer, to take advantage of repulsive electrostatic interactions. However, there are two common drawbacks to this approach: the alteration of the permeability/selectivity trade-off and the increased fouling propensity of positively charged membranes towards negatively charged organic foulants, which should be overcome for effective membrane utilization. To overcome this, positively charged aminated carbon quantum dots (CQDs) were incorporated into a positively charged selective layer to maintain selectivity against metal cations. CQDs incorporation improved membrane hydrophilicity, affected pore size distribution and molecular weight cut-off and smoothed the surface of the selective layer. As a result, membrane permeance increased by 2.3 times, up to 12 l/(m²·bar·h), compared to the pure membrane, while the positive surface charge contributed to maintaining high rejection rates for double charged cations: 92.6 % for MgCl₂ and 88.5 % for CaCl₂, and with a slight reduction of NaCl rejection from 56.5 % to 49 %. The fabricated membranes were tested for softening feed solutions simulating realistic brackish water and seawater compositions. For brackish water with total dissolved solids up to 6000 ppm, the rejection rates for Mg²⁺ and Ca²⁺ ions exceeded 93 % and 87 %, respectively, achieving total water hardness removal higher than 90 % and a Mg²⁺/Na⁺ separation factor up to 14, which can be utilized for pretreatment of brackish and sea water before the RO desalination process. Furthermore, the modification enhanced membrane antifouling properties due to improved membrane hydrophilicity and reduced surface roughness. In summary, incorporating aminated positive CQDs is an effective method for enhancing the characteristics of positively charged nanofiltration membranes for water softening as pretreatment for brackish water and seawater desalination.