Plastic is a material that has become ubiquitous since entering the marketplace in the 1930s and 1940s; as a result, the presence of nano and microplastics (NMPs) are pervasive in natural environments affecting air, soil and water ecosystems. These NMPs are varied in size (categorised as either microplastics at 5 mm–1 µm or nanoplastics at < 1 µm), shape and chemical composition. They represent a potential threat to aquatic life and human health through ingestion and inhalation. The toxicity of NMPs is attributed to chemical additives introduced during production and the absorbance of inorganic and organic chemical contaminants in environmental settings. This review is designed to discuss the use of biochar as a natural adsorbent for the remediation of water contaminated with NMPs. Biochar is a sustainable, affordable material which can remediate water and contribute to ecosystem restoration. Whilst it is well established as a material to sorb organic and inorganic contaminants, its use to remove NMPs is in its infancy and as such this review sets out to outline the mechanisms and modifications of biochar to remove NMPs from aqueous environments. Although removal mechanisms in laboratory settings are becoming clearer this review highlights that remediative studies need to be undertaken in conjunction with the systematic investigation of the effect of key environmental parameters on remediation and the use of environmentally aged NMPs. The future direction of this discipline also needs to incorporate field trials alongside laboratory work to develop a stronger understanding of the viability of biochar to remove NMPs from waterways.

A widely recommended strategy for releasing fish that have ingested hooks is to simply cut the line. The utility of this approach is based on the premise that the individual will eventually eject the hook following sufficient oxidation. However, few quantitative data are available describing the mechanisms affecting hook decay. We addressed this issue by testing the independence of various technical factors on the degradation of 828 hooks comprising 23 designs (absolute sizes 227–611 mm2) after protracted submersion in seawater. Twelve replicates of each hook were destructively assessed for compression and tensile strengths (using a force gauge) and 24 replicates were weighed, photographed and submersed in seawater. After submersion for 8 and 28 days, 12 replicate hooks were removed, re-photographed, re-weighed and tested for compression and tensile strengths to provide indices of decay. Hook degradation was mainly affected by the wire material and diameter and could be significantly promoted by choosing carbon steel designs, either with a wire diameter of ≤0.9 mm for the examined sizes or, alternatively, bait-holder barbs (or similar modifications) along the shaft. By rapidly oxidising and weakening after ingestion, such designs could ultimately help to reduce negative impacts of hooks on released fish.

Lead and zinc mines are a primary source of environmental (post)-transition metal contamination resulting in major water pollution. In this study, the use of biochar amended with wood ash (WAS) was evaluated as a method to remediate zinc and lead contaminated mine water. Water from Nantymwyn lead mine, with zinc and lead concentrations as high as 12.1 mg L⁻¹ and 1.7 mg L⁻¹ respectively was used. The contact time for WAS to immobilise zinc and lead (1 min to 24 h), immobilisation mechanisms and maximum measured removal of lead and zinc were studied. FTIR spectroscopy and XPS was used to characterise WAS and the aqueous modelling program PHREEQC (pH redox equilibrium) was used to analyse mine water speciation. The fast removal performance of a biochar is a key indicator of its viability to be used as a green remediator. If the required contact time to remediate contaminated water is too long the sorbent becomes impractical. This study demonstrated that WAS removed 97% of zinc and 86% of lead within the first minute of contact with the mine water (0.5 g of WAS per 20 mL of mine water), with a maximum measured removal of 14.8 mg g⁻¹ for zinc and 23.7 mg g⁻¹ for lead (using 0.1–0.002 g of WAS per 40 mL of mine water). Fast removal was primarily a result of precipitation, and subsequent capture by WAS, and ion exchange. These findings show that WAS has the potential to be scaled up and deployed at mine sites to remediate contaminated water.

The pollution of aqueous environments by metals has continued to increase due to anthropogenic activities such as mining, waste disposal, industrial activities and the use of motor vehicles. Globally, vehicle numbers are predicted to increase to 2.8 billion by 2050 with aqueous pollutants associated with vehicles, such as Pb, Cu, Zn and Cd, increasing alongside vehicle numbers. With these increases, methods to minimise metal pollution are important to find; one such method is wood ash amended biochar. Whilst biochar has the potential to reduce contaminated runoff, this study explores the potential for the biochar itself to leach nutrients (PO₄ ³⁻, SO₄ ²⁻ and NO₃ ⁻) which are constituent parts of its biomass and potentially harmful to the ecosystem the biochar would be deployed to remediate. Treatments such as sintering wood ash to the biochar, granulating the biochar and rinsing the biochar were studied to ascertain their impact on the retention of minerals key to immobilisation, the leaching of nutrients and the immobilisation of Pb, Cu, Zn and Cd. It was demonstrated that wood ash sintered larch biochar granulated <3 mm (WASGr) retained the highest concentration of minerals associated with immobilisation, reduced leaching of nutrients to below Water Framework Directive thresholds and maintained Pb, Cu, Zn and Cd immobilisation at 97–100% once rinsed with deionised water. As a result, WASGr rinsed with deionised water, has the potential to be scaled-up and deployed to immobilise Pb, Cu, Zn and Cd from motorway runoff without a negative impact on the concentration of nutrients in the surrounding waters.

Microplastics and metals are increasingly recognised as major water contaminants with profound environmental and health consequences. The environmental co-occurrence of microplastics and metals are well documented in waterways, including urban runoff, highway balancing ponds, industrial wastewater, and mine-impacted waters, posing a multifaceted environmental threat. Urgent remedial action is required to remove co-occurring microplastics and metals from water, giving consideration to how their co-occurrence can affect remediative efforts. However, information on the sorption of microplastics and Pb and Zn simultaneously by biochar is lacking. In this current study, changes in the quantity of metal adsorbed by pristine larch biochar and magnetised larch biochar due to the presence of microplastics was assessed using spectroscopic techniques. This study demonstrated that magnetised larch biochar and pristine larch biochar both remove co-occurring microplastics, Pb, and Zn from solution. Neither magnetised larch biochar nor pristine larch biochar show any statistical difference in the sorption of Pb with the inclusion of microplastics into the aqueous matrix. However, the inclusion of microplastics result in the reduced sorption of Zn by 43% for magnetised larch biochar (p < 0.01) and 69% for pristine larch biochar (p < 0.01). Magnetised larch biochar also demonstrated greater sorption than pristine larch biochar for microplastics (p < 0.05), Zn co-occurring with microplastics (p < 0.05), and Zn with no microplastics present (p < 0.01). Despite the effects of competitive sorption between Zn and microplastics, the removal of Pb, Zn, and microplastic from a multi-contaminant system indicate that magnetic larch biochar is a viable option to remove multiple contaminants from aqueous environs where metals and microplastics are seen to co-occur.