In recent years, biochar has emerged as a remarkable biosourced material for addressing global environmental, agricultural, biomedical, and energy challenges. However, the performances of biochar rest in part on finely tuning its surface chemical properties, intended to obtain specific functionalities. In this review, we tackle the surface treatment of biochar with silane and other coupling agents such as diazonium salts, titanates, ionic/non-ionic surfactants, as well as nitrogen-containing (macro)molecules. We summarize the recent progress achieved mostly in the last five years and correlate the nature and extent of functionalization to eye-catchy end applications of the surface-engineered biochar.

The production of porous biochar is highly needed for numerous applications such as depollution and catalysis. It rests on a variety of eco-friendly physical and chemical process-assisted fabrication of porous biochar via pyrolysis under an inert atmosphere. The objective is to compare the features of the porous biochar powder particles prepared via such activation routes. The low-cost biochar with abundant pore structures was obtained from palm waste fibre. The pyrolysis temperature was set to 800 ℃ based on the boiling point of ZnCl2 and increasing more macro-, meso- and microporosity of palm waste biochar (PWB). The physicochemical and textural properties of all PWBs were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), point of zero charge (pHPZC), and methylene blue index (MBi) and iodine index (Ii). ZnCl2- and H3PO4-activated PWBs showed more pore and channel structures than C2H5OH-activated and ultrasonicated PWBs, as noted from SEM images. Raman results proved that the I(D)/I(G) ratio values of all PWBs ranged from 2.19 to 3.19, confirming they are typical amorphous crystalline carbons as confirmed by XRD. The pHPZC values of all PWBs fell within the acidic range of 2.01 to 6.75, indicating the PWBs favor the cationic substance adsorption in an aqueous solution with a pH greater than 6.75. The process with ZnCl2 and H3PO4 significantly enhanced the macroporosity of the PWBs based on MBi. The MBi values increased with the increase in the ZnCl2 mass ratio, from 16.3 to 100.0 mg/g. As judged from the Ii values, the change of the ZnCl2 mass ratio had a relatively small effect on the formation of microporosity, with the microporosity slightly increasing with the initial ZnCl₂ mass ratio, from 920.6 to 1066.8 mg/g. However, the microporosity decreased more significantly with C2H5OH-, H3PO4-processed, and ultrasonic samples, dropping from 920.6 to 536.2 mg/g, compared to ZnCl2-activation. The BET results showed that ZnCl2 and H3PO4 noticeably increased the specific surface area and total pore volume of PWBs, from 392.6 to 1556.7 m2/g and 708.3 m2/g for specific surface area, from 0.186 to 0.741 cm3/g and 0.667 cm3/g for total pore volume. This work permitted to obtaining of a series of different types of porous biochar through simple and inexpensive physical and chemical treatments to provide a reference value for producing low-cost biochar for water treatment and other applications.