Liquid–Liquid membrane contactors incorporating surface skin asymmetric hollow fibres of poly(4-methyl-1-pentene) for ammonium recovery as liquid fertilisers

Abstract

Two novel hollow-fibre liquid–liquid membrane contactor (HF-LLMC) modules, containing S-type (skin layer with low porosity) and Q-type (skin layer with higher porosity) fibres, have been evaluated. Both fibres, with asymmetric, porous, and hydrophobic membranes, made from poly(4-methyl-1-pentene) (PMP), were used as efficient technology for ammonia recovery to produce liquid fertilisers. The ammonia-rich stream was fed into the shell side of the PMP-HF-LLMC, while nitric acid or phosphoric acid were fed separately into the lumen to produce N-type (NO3–-NH4+) and N-P-type (NH4+-P2O5) fertilisers, respectively. The maximum NH3 recovery (>95%) was achieved in a closed-loop configuration with S-type fibres, while Q-type fibres showed a better performance in terms of the production of more concentrated N-type liquid fertiliser. With Q-type fibres, the highest values of N-P-type liquid fertilisers were achieved (8.0 % N (NH4+) and 20.3% P2O5 (w/w)) using phosphoric acid, while the highest value of water flux across the PMP fibres was < 0.01 kg m–2h-1. The highest overall mass transfer coefficient (Km), measured for solutions containing 5.0 gNH3 L–1 with a feed/stripping volume ratio of 60:1, was (2.9 ± 0.2) × 10-7 m s-1. Additionally, ultraviolet–visible spectroscopy and two-dimensional excitation-emission matrix fluorescence spectroscopy were employed to monitor the absence of pore-wetting events and the stability of the PMP-HF-LLMC under strongly acidic and basic conditions.