Synthesis, characterization and application of magnetic and acid modified biochars following alkaline pretreatment of rice and cotton straws

The development of distinct biochar from agricultural waste for soil and environment remediation is valuable. Moderate pretreatment with sodium hydroxide may open the lignocellulosic structure of crop straw and then enhance the impregnation of iron oxides and phosphates, finally leading to the production of distinct biochars. In this study, two common agricultural wastes of rice and cotton straw were first treated with a dilute NaOH solution and then soaked in either Fe-Co nitrate or H3PO4 solution. The biochars produced through a slow pyrolysis process were then analyzed with respect to their physico-chemical and adsorptive properties. The results showed that all pretreatments remarkably changed the physico-chemical properties of the feedstocks and subsequently endowed the biochars with distinct characteristics. The biochars had specific surface areas (SSAs) ranging from 12.26 to 581.13 m2/g, total pore volumes (TPVs) ranging from 0.033 to 0.3736 cm3/g and average pore volumes (APSs) ranging from 2.57 to 10.76 nm. They also contained a large amount of positive charge, an anion exchange capacity (pH 3.5) ranging from 251.78 to 810.13 mmol/kg, and a certain amount of negative charge as well, cation exchange capacity (pH 7.0) ranging from 108.22 to 464.67 mmol/kg. The adsorption capacities of the modified biochars toward both Pb2+ and Cd2+ were 23.07–82.74% and 16.90–556.33% higher than those of pristine biochars, respectively. Of the modified biochars, the Fe-Co-composite biochar showed many promising physico-chemical and adsorptive properties for adsorbing divalent metals of both Pb2+ and Cd2+ and might thus have high potential as a soil amendment and an alternative adsorbent for environmental remediation.