Preparation and Application of Biochar-Chitosan Magnetic Composite Adsorbent for Removal of Lead and Copper from Groundwater

Wastewater containing heavy metals produced by mining, beneficiation, smelting, forging, processing, transportation, and other industries that has been improperly disposed of, leads to heavy metals entering and polluting the groundwater environment. Heavy metals can be enriched in the human body and participate in the biological cycle. Long-term accumulation of heavy metals in the human body will bring carcinogenic, teratogenic, and mutagenic risks. Adsorption has been of wide concern in the treatment of heavy metals pollution in water, due to the advantages of low operation cost, simple engineering operation and low secondary pollution. Chitosan as a natural organic polysaccharide organic matter, has the characteristics of being environmentally friendly. It contains many nitrogen-containing functional groups that can adsorb metal ions in water. However, the adaptability of chitosan adsorbents to acidic conditions is poor, so the pH value needs to be adjusted in the actual process, which increases the operating cost. Combining biochar with chitosan can not only improve the adsorption capacity of chitosan, but also improve the separation performance of biochar. However, most of the research on chitosan modified biochar concentrate on the single biochar. There are few studies on the modification of biochar from different sources by chitosan, and the interaction between chitosan and biochar is not clear.

The aim of this study was to prepare peanut shell biochar-chitosan magnetic composite adsorbent (PSC) and corn cob biochar-chitosan magnetic composite adsorbent (CCC), and to investigate the Pb²⁺/Cu²⁺ adsorption properties and mechanisms on PSC and CCC.

Scanning electron microscope was used to analyze the microstructure of the material, and the material samples were treated with gold spray before photographing. The specific surface area and pore volume of the material were determined using a specific surface area analyzer, and the material was adsorption-desorption tested with nitrogen at -196℃. X-ray diffraction was used to analyze the crystal structure of materials, and the Cu Kα source was used to scan in the range of 10°-80° (2θ). X-ray photoelectron spectrometer was used to analyze the changes of functional groups on the surface of the material, and the radiation (225W, 15mA, 15kV) was carried out by monochromatic Al-Kα. Metal ion content in solution was measured by inductively coupled plasma-optical emission spectrometry.　　The adsorption experiments were carried out in a 50mL conical flask and a constant temperature shaker. The oscillation frequency was 150r/min and the reaction time was 12h. After the reaction, the concentrations of Pb²⁺ and Cu²⁺ in the solution were determined. Two parallel samples were set up in each group. Different initial pH experiments were used to evaluate the adsorption performance of materials. Kinetic and isothermic models were used to evaluate the adsorption kinetic process of materials and predict the maximum adsorption capacity of materials. Recycling experiments and actual mine groundwater adsorption experiments were used to evaluate the practical application capacity of materials.

SEM images of PS and CC showed that there were many pores on the surface of both types of carbon. There were more pores and bulges observed on the surface of CC than PS, indicating that CC had a larger contact area with pollutants than PS. The specific surface area (12.045m²/g) and mean pore diameter (3.614nm) of CC were larger than those of PS (specific surface area was 3.294m²/g and mean pore diameter was 3.067nm), which was consistent with the SEM results. The specific surface area (4.598m²/g) and average pore diameter (3.417nm) was 2.812nm), indicating that the primary structural properties of biochar affected the pore structure of the composites. SEM of CCC and PSC showed that chitosan and biochar were well combined. Compared with previous literature, the biochar-chitosan composite material in this study preserves the original structure of biochar to the greatest extent, and makes the chitosan evenly coated on the surface of biochar. The results BET showed that the primary structural properties of the biochar affected the physical properties of the modified materials. The XRD results showed that Fe₃O₄ was successfully embedded into the composite material. Three pH values (4, 7 and 10) were selected to evaluate the swelling properties of the materials. The results showed that the swelling ratios of the two materials were similar under the three pH conditions, and neither of them was more than 100.0%, which was due to the biochar as a carrier having a good supporting effect. Compared with chitosan/kiwifruit branch biochar adsorbent, the two adsorbents in this study showed relatively stable adsorption properties in the pH range of 4-7, indicating that the adsorbents had a wider range of pH value application.　　The effect of the initial pH on the adsorption was tested. As the initial pH value increased from 3 to 7, the adsorption capacity and removal efficiency of the material increased. The positive charge of the adsorbent surface also decreased, which reduced the electrostatic repulsion of Pb²⁺ and Cu²⁺ between the material surface and the solution, and increased the electrostatic attraction between the material surface and Pb²⁺ and Cu²⁺. The increasing electrostatic attraction was beneficial to the adsorption of Pb²⁺ and Cu²⁺. When PSC and CCC adsorbed Pb²⁺, the pseudo-first-order kinetic model could better describe the adsorption process than the pseudo-second-order kinetic model, indicating that physical adsorption played a dominant role in the adsorption of Pb²⁺ by PSC and CCC. Pseudo-second-order kinetics can better fit the Cu²⁺ adsorption by PSC and CCC than pseudo-second-order kinetic model, indicating that chemisorption dominates the adsorption process of Cu²⁺ by PSC and CCC. The Langmuir isotherm adsorption model can better describe the adsorption process of Pb²⁺ and Cu²⁺ than Freundlich isotherm adsorption model, indicating that the adsorption process is monolayer adsorption.　　The EDTA-2Na was used as the desorption agent of chitosan composite, the removal efficiencies of Pb²⁺ and Cu²⁺ were still above 85% after five cycles, which indicated that the two adsorbents in this study had excellent stability. PSC and CCC were the low-cost and effective adsorption materials. Groundwater of an acid mine was taken from a mining area in Dongshan District, Dafan Mountain, Anhui Province. The groundwater samples contained large amounts of metal ions, such as As, Ca, Cd, Cu, Fe, Mn, Na and Ni, the corresponding concentrations were 18.200g/L, 40.541mg/L, 3.800g/L, 13.300mg/L, 215.00mg/L, 510.00g/L, 81.694mg/L and 87.000g/L, respectively. 0.45μm filter membrane was used to remove particulate impurities from the water before adsorption. CCC was used to treat the heavy metal polluted groundwater, and the results showed that CCC has a good removal ability on a variety of metal ions in groundwater such as Cu²⁺, Cd²⁺, and Fe³⁺. The Cu²⁺ concentration in the treated water can reach the Grade IV water standard of the “Groundwater Quality Standard”.　　The pH results showed that the removal mechanisms of Pb²⁺ and Cu²⁺ by PSC and CCC mainly included electrostatic attraction. The functional groups of adsorbents before and after adsorption were analyzed through X-ray photoelectron spectroscopy (XPS). The results of XPS showed that complexation was another removal mechanism. The nitrogen (—NH) of pyrroles, amino of chitosan (—NH₂), and C=N were the main functional groups, which were responsible for the complexation with Pb²⁺ and Cu²⁺.

The preparation method of the materials in this study can effectively improve the adaptability of chitosan materials under different pH conditions. The adsorbents developed in this study can effectively remove heavy metals from groundwater and have a good application potential.