# Low-Cost Biochar Adsorbents for Water Purification Including Microplastics Removal
## 5 Conclusions  
Both spruce and pine bark were suitable for biochar and activated biochar production for water treatment purposes, but some differences were observed between the two materials. The surface area of spruce bark AC remained less than the surface area of pine bark AC. One of the reasons for this was likely due to the high ash content of spruce bark, causing pore-clogging through carbonate formation. The lower carbon content of spruce bark also indicated that longer activation time or greater steam proportion may have yielded a higher degree of carbonization and greater porosity development. Pine bark activated using 30% steam and the higher rate of gas flow performed best in the phenol adsorption tests and had relatively greater CEC due to its larger surface area. Spruce ACs possessed a greater quantity of exchangeable cations, largely due to a high Ca content. The activated biochars  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Raw Material** & **Activation Condition** & **Surface Area \(\mathbf{m^{2}/g}\)** & **Phenol Adsorption Capacity** & **Reference** \\ \hline Spruce and pine bark & Steam & 185–615 & 64–169 mg/g (=0.7–1.8 mmol/g) & Current work \\ Cherrystone & KOH and ZnCl\({}_{2}\) & 170–465 & Max \textasci{-}70 mg/g for KOH and ZnCl\({}_{2}\) & [39] \\ Eucalyptus seed & ZnCl\({}_{2}\) & 250–300 & 200 mg/g & [38] \\ Switchgrass (SG), & & & & \\ hardwood (HW) and & Steam & 167–383 & 1.0–1.6 mmol/g & [15] \\ softwood (SW) & & & & \\ Ratan sawdust & KOH & 1083 & 149 mg/g & [40] \\ Oil palm shells & Steam & 988 & 166 mg/g & [17] \\ Rapeseed and & & & & \\ raspberry seed cakes & Steam, CO\({}_{2}\) & 141–1179 & Max \textasci{-}250 mg/g steam, \textasci{-}170 mg/g CO\({}_{2}\) & [18] \\ Fir woods and & & & & 2.58 and 2.72 mol/kg steam, 2.74 and & \\ pistachio shells & Steam, KOH & 1009–1096 & 3.03 mol/kg KOH & [41] \\ \hline \hline \end{tabular}
\end{table}
Table 7: Comparison of phenol adsorption results of current work to literature.  
tested herein efficiently removed the larger microplastics particles. The removal of 10-\(\upmu\)m spherical microplastics was not as sufficient. Results indicated that higher meso-and macroporte contents could be beneficial for the removal of the smallest MP particles. In conclusion, steam activation is a suitable method for activated biochar production. Surface areas in the range of 400-600 m\({}^{2}\)/g are adequate for the efficient removal of contaminants from storm and wastewater. The results support the economic feasibility of steam-activated biochar for such water purification purposes. Carbonization provides an added value use for the ligno-cellulosic forest residues with environmentally benign applications. Removal of microplastics using biochar requires further research, particularly regarding the recovery of micrometer-scale MP size fractions as well as the identification of the retention mechanism. In addition, the economic feasibility of biochar production using different sidestreams or combinations thereof should be further considered in the biochar and activated biochar-related research.  
[PERSON] and [PERSON] conceptualized and designed the research topic; [PERSON] and [PERSON] supervised the investigation; [PERSON], [PERSON], [PERSON], and [PERSON] designed and executed the laboratory work; [PERSON] prepared the original manuscript draft; all authors contributed to reviewing and editing the manuscript. All authors have read and agreed to the published version of the manuscript.  
This work has been funded by the Ministry of Agriculture and Forestry of Finland, agreement number: 754/03.02.06.00/2018.  
The authors are grateful to [PERSON] (Oulu University, Finland) for performing the phenol adsorption tests and to [PERSON] (VTT) for extensive support with the slow pyrolysis oven. We thank the laboratory staff at VTT for performing the analyses. We are grateful for the support by the FinnCERES Materials Bioeconomy Ecosystem.  
The authors declare no conflict of interest.