In this work, the adsorption capacity of the biochar obtained from Pinus patula biomass micro-gasification was studied using malachite green (MG) as the probe pollutant. For this purpose, the biomass type (wood pellets and chips) was selected to produce two kinds of biochar (BC). Afterwards, the effects of the adsorbent dose (6, 9 and 12 g/L), the solution pH (4, 7 and 10) and the BC particle size distribution (150–300, 300–450 and 450–600 μm) for the maximization of the MG retention by the selected BC were evaluated using a faced-centered central composite design, as response surface methodology. The results indicated that the BC derived from wood chips (BWC) exhibited a higher MG dye adsorption capacity than the BC obtained from the wood pellets (BWP) gasification under the same operating conditions after having reached the equilibrium. A second-order regression model was built for describing the MG adsorption behaviour by BWC under the considered experimental domain. The model, which was validated, resulted to be statistically significant and suitable to represent the MG adsorption by the studied BC with a p-value of 0.00 and a correlation coefficient (R2) of 95.59%. Additionally, a three-dimensional response surface graph and a contour plot were utilized to analyze the interaction effects between the factors influencing the adsorption system and to discern the optimal operating conditions for the use of BWC. The maximal MG dye retention (99.70%) was found to be at an adsorbent dose, pH solution and a particle size distribution of 9.80 g/L, 10 and from 150 to 300 μm, respectively. Therefore, the BWC tested can be utilized for the treatment of water polluted with dyes, contributing to the establishment of a circular economy. © 2021 King Saud University