Cryogel-supported titanate nanotubes for waste treatment: Impact on methane production and bio-fertilizer quality

By reducing the cadmium (Cd2+) content in biomass used for bio-based products such as biogas, a less toxic bio-based fertilizer can be obtained. In this work, we demonstrate how a macroporous polymer can support titanate nanotubes, and we take advantage of its known selective adsorption behavior towards Cd2+ in an adsorption process from real nutrient-rich process water from hydrolysis of seaweed, a pollutant-rich biomass. We show that pretreatment steps involving alteration in area-to-volume ratio performed in aerated and acidic conditions release the most Cd2+ from the solid material. By integrating an adsorption step between hydrolysis and the biomethane, we show that it was possible to obtain high Cd2+ removal (ca. 94%) despite molar excess (between 100 and 500) of co-present ions (e.g., Mg2+, Ca2+, Na+, K+) and with maintained total phosphorous content. The bio-methane potential did not significantly decrease as compared to a process without cadmium removal and the yielded bio-fertilizer followed Swedish guideline values. This study provides a sound and promising alternative for a novel remediation step, enabling higher use of otherwise tricky and to some extent overlooked biomass sources.