Abstract
Rice straw is a kind of low-cost biosorbent. Through mechanical crushing, pyrolysis, incineration, and citric acid (CA) modification, it could be converted to rice straw powder (Sp), biochar (Sb), ash (Sa), and modified rice straw (Ms) accordingly. Using rice straw as an adsorbent, the influence of pH value (2, 4, and 9), initial Cd(II) concentration (0, 200, and 800 mg/L), and ionic strength (0, 0.2, to 0.6 mg/L) on the adsorption capacity for Cd(II) were examined with three replicates, and the relevant mechanisms were explored using Fourier transform infrared (FTIR) technology. Results showed that the modifications could improve the adsorption capacity of Cd(II) by changing their chemical structures. The products (Sb and Sa) of the pyrolysis and incineration of rice straw contained fewer hydroxyl and alkyl groups, but more Si–O groups. Citric acid modification removed a portion of silica in rice straw, increased its carboxylic content, and made more –OH groups exposed. Compared with Sp, Sb, Sa, and Ms were more likely to act as π donors in the Cd(II) sorption process and exhibited more carboxyl binding. The bands of C = C, –O–CH3, and the O–H, carboxyl, Si–O–Si or Si–O groups were involved in the Cd(II) sorption process. The adsorption amount of Cd(II) by the four adsorbents increased with the increase in the pH value of the solution and the initial Cd(II) concentration. Affected by pH in a solution, ion exchange, surface complexation, and precipitation were the major adsorption mechanisms. Further, under the influence of the initial Cd(II) concentration, electrostatic attraction played a leading role. With no interference by ionic strength, all the adsorbents had the greatest adsorption amount of Cd(II), and the intensity of O–H vibration was also the weakest; ion exchange was the most important mechanism in this process. Regardless of the influencing factors, Sa, with the greatest specific surface area, had an absolute advantage in the adsorption capacity of Cd(II) over Sp, Sb, and Ms.
Highlights
Cadmium (Cd) is one of the most toxic heavy metal elements
(10 mm), pulverized with a grinder, and passed through a 0.25 mm sieve; rice straw ash (Sa): the rice straw powder was placed in a thin stainless steel tray, and the bottom of the tray was burned with an alcohol lamp while continuously turning over the straw until fully burned; rice straw biochar (Sb): the rice straw powder was placed in a 700 mL ceramic crucible, covered with a lid, and pyrolyzed in the absence of oxygen using a muffle furnace
The rice straw biochar, ash, and modified rice straw were more likely to act as π donors in the Cd(II) sorption process [1]
Summary
Cadmium (Cd) is one of the most toxic heavy metal elements. Cd(II) has become a significant concern owing to its high mobility and biological accumulation. Chemical modification of rice straw with citric acid, phosphoric acid, etc., can enhance the number of surface carboxyl groups and increase the cation exchange capacity of the material [12,13]. The functional groups such as carboxyl, silanol, etc., present in rice husk ash are favorable for Cd(II) adsorption [14]. The modification has the potential of transforming rice straw into a more valuable adsorbent to remove heavy metal pollution in water [10]. The soybean straw was citric acid (CA) modified to introduce high amounts of free carboxyl groups and enhance its adsorption capacity of Cu(II) [8]. The objectives were to: (1) determine the adsorption capacity of Cd(II) on the four adsorbents in aqueous solution under the different influencing factors, and (2) understand the mechanisms of Cd(II) removal from water by the four adsorbents on a qualitative and quantitative basis
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