# Low-Cost Biochar Adsorbents for Water Purification Including Microplastics Removal  
[PERSON] 1,*  
1 VTT Technical Research Centre of Finland Ltd., P.O. Box 1000, 02044 Espoo, Finland; 1 [EMAIL_ADDRESS] (L.W. ); 2 [EMAIL_ADDRESS] (P.K. )1 VTT Technical Research Centre of Finland Ltd., P.O. Box 1000, 02044 Espoo, Finland; 1 laura. purification. For profitable production of a bio-adsorbent, both the raw material and the treatment process need to be low-cost. Chemical activation can be used to produce ultrahigh surface areas and porosities because of extensive microporosity development. High micropore volume promotes adsorption of particularly small-sized metals and molecules. Still, a broader pore structure plays a vital role in adsorption processes. The small micropores that are accessible to nitrogen molecules in surface area measurements may not be accessible for contaminant molecules in solution. Meso- and macroscale pores are essential as vectors to areas deeper within the biochar particle, and their respective quantities are primarily dependent on the raw materials used in biochar production. In our previous study [4], 3D-modeling of pine bark biochar and phosphoric acid activated pine bark revealed that chemical activation did not affect the micrometer scale porosity of the activated biochar. Also significant are the elevated costs arising from the use of chemicals and intensive washing procedures [5]. Producing chemically activated carbons for wastewater treatment may be uneconomical because of the large quantities of chemicals needed and reuse of the used activated carbons may not be possible. The main applications for chemically activated carbons should be in higher value products such as supercapacitors, where the surface area and specific pore size distribution are critical parameters for their functionality [6; 7]. Thermal treatment of biomass can be divided into three different paths: torrefraction, gasification, and pyrolysis [8]. The most important differences are the residence times and temperature gradients used, particle size of feedstock materials, and the distribution of products into gas, pyrolytic liquids, and solid materials. The pyrolysis of biomass can further be divided into fast, medium, and slow pyrolysis; of these the fast and slow pyrolysis are the ones mostly used. Fast pyrolysis with a residence time of seconds is used for the production of liquids whereas slow pyrolysis with residence times of minutes to hours is used to produce chars. Characteristics of the individual processes are summarized in Table 1.  
The chars obtained from slow pyrolysis can undergo further physical or chemical treatment to generate activated carbons. Physical activations using CO\({}_{2}\) or steam eliminate the need for chemicals and subsequent washing procedures. The use of steam minimizes the activation chemical costs and promotes the formation of larger pores in the activated carbon (AC), although the resulting porosity is also dependent upon characteristics of the feedstock raw material [9]. The adsorption efficiency is related to the surface functionalities of the adsorbent carbon where, for example, a relatively large number of oxygen groups, enhance adsorption of cationic contaminants. The total number of surface functional groups in physically activated carbons are usually less than for chemically activated carbons because of the higher temperatures used. Critical views for their suitability for water treatment have been presented elsewhere [5]. Activated biochars possessing sufficient surface area and suitable porosity for tertiary wastewater purification that can be produced economically are of widespread interest [3; 10; 11]. The availability of bio-based feedstock is an essential variable for biochar and activated carbon (AC) production. Large quantities of lignocellulosic sidestreams suitable for biochar production, such as sawdust and bark residues, are generated by the forest industry.