Comparative Time-Dependent Analysis of Heavy Metal Rejection from Mine Water Using Ultrasound-Enhanced Forward Osmosis

Acidic mine water, contaminated with toxic heavy metals such as Pb, Zn, and Fe, poses significant environmental risks. This study investigated the application of ultrasonic irradiation in forward osmosis (FO) to enhance the simultaneous enrichment and rejection metals. The effects of key operational parameters were evaluated, including the draw solution concentration (1 and 2 M), pH (4, 7, and 9), ultrasonic frequency (28 and 40 kHz), ultrasonic mode (continuous and pulsed), and membrane orientation. The results demonstrated high heavy metal rejection efficiencies, reaching 99.5% for Fe, 99.26% for Pb, and 88.59% for Zn. Rejection performance was frequency-dependent: 40 kHz was more effective for Fe and Pb, while 28 kHz yielded better Zn rejection. Although metal retention generally declined over time because of operational factors, Fe maintained consistent performance throughout filtration. The rejection order across both ultrasonic modes was Fe>Zn>Pb. Differential scanning calorimetry revealed that ultrasonication at 28 kHz resulted in the lowest glass transition temperature (Tg=39.33°C), corresponding with the reduced average water flux observed in the US-FO-28 condition. Ultrasonication also decreased the crystallinity index from 46% to 42%. Additionally, atomic force microscopy confirmed reduced solid deposits on the membrane surface under 28 kHz vibrations, evidenced by a decrease in root mean square roughness (Rq). Overall, the high metal rejection efficiencies and favorable membrane properties demonstrate that ultrasonic irradiation enhances FO performance, positioning it as a promising and sustainable alternative for the treatment of heavy metal-laden mine water.