# Low-Cost Biochar Adsorbents for Water Purification Including Microplastics Removal
## 4 Discussion
### Raw Material Selection and Particle Size  
The raw materials selected for these experiments were two soft-wood bark materials, which were not entirely homogeneous as some stem wood was also mixed in the samples. The effect of the stem wood was, however, likely minimal as the quantities of entrained stem wood were small. The main differences between stem wood and bark are different porosities and amounts of ash. The porosity of the biomass affects the porosity of the biochar and activated biochar [4]. Bark material also has higher ash content than stem wood [33; 34]. Mineral impurities could have caused the high ash content of spruce, but the freshness and storage method of the material also affect the ash content [33]. The spruce bark biomass was more fresh and moist compared to the pine bark, which had been stored in dry conditions for a longer time period.  
Both bark materials produced microporous ACs, but differences were found in the larger pore sizes. Spruce bark biochar had a higher amount of mesopores before activation, and the pores were enlarged during the activation into macropores. Pine bark biochar did not increase in macropore volume due to activation, but mesoporosity increased by ten-fold. It has been shown that steam activation produces more mesoporous carbons than CO\({}_{2}\) activation [35; 36]. Steam reacts more readily with the carbon pore walls and begins to expand the existing pores, whereas CO\({}_{2}\) creates microporosity by reacting primarily with the active sites at the pore centers [35]. Longer activation times are needed with CO\({}_{2}\) if larger pores are required. The results reported herein are supported by those of [PERSON] et al. (2014) who used poplar wood bark and [PERSON] et al. (2002), who detailed the outcomes of steam activation of poplar wood bark and a mixture of soft wood bark, respectively [19; 37]. Both materials produced microporous ACs with similar surface areas (e.g., 555 m\({}^{2}\)/g for poplar bark at 800 \({}^{\circ}\)C and 60 min). The observed differences in the results are due to the different raw material porosity, activation times, and activation temperatures. The development of mesoscale porosity is important to take into account when selecting raw material and activation method for water purification purposes. The adsorption occurs mainly in the micropores, but larger pores work as channels into the micropores. Bark materials are therefore suitable for AC production as they possess mesoporosity and the required microporosity can be generated by activation.  
The biochar particle size before activation did not significantly affect the surface area and porosity. Although small particle size generally favors porosity development because of the larger reactive surface area for the activation reagent, crushing is an extra step in the manufacturing process. Surface area as high as 1361 m\({}^{2}\)/g has been achieved for crushed walnut shells (particle size 1-2 mm), for example, using steam activation at 850 \({}^{\circ}\)C and 60 min [35]. When aiming to produce ACs with moderate surface areas, however, sufficient results can be achieved with larger biochar particles.