Abstract:

The removal of cadmium (Cd) from water using modified biochar is a growing area of research. However, studies on the adsorption mechanism of Cd on biochar at the micro-level using Surface Complexation Models (SCM) are scarce. This study investigated the effectiveness of magnesium-modified wheat straw biochar (MgWB) for cadmium (Cd²⁺) adsorption and explored the underlying mechanisms using SCM. MgWB exhibited a 3.97% increase in the Mg mass fraction and a 9.94% decrease in the specific surface area compared to those of unmodified biochar (WB). The isothermal adsorption curves revealed the adherence of biochar to the Langmuir model before and after modification. The maximum adsorption capacity of MgWB (3.50 μmol·m⁻²) at 313 K was found, 3.7 times higher than that of WB. Boehm titration showed a 13.4% increase in total acidic functional groups upon modification. The high-, medium-, and low-affinity functional groups, represented by hydroxyl, lactone, and carboxyl groups, increased by 4.28, 8.92, and 0.178%, respectively. Potentiometric titration curves demonstrated a reduction in the biochar's pH_pzc from 9.2 to 5.3 with magnesium addition. Furthermore, the pH required for 90% adsorption of WB and MgWB in Cd²⁺ adsorption experiments was observed at pH 9 and 6, respectively, suggesting a correlation between the adsorption equilibrium pH and pH_pzc alterations, impacting Cd adsorption by biochar surface sites across varying pH conditions. Non-electrostatic surface complexation modeling (NEM) revealed that high-affinity sites dominate Cd²⁺ adsorption for both WB and MgWB. MgWB exhibited a lower pH for peak adsorption than WB, indicating stronger complexation with Cd²⁺ owing to the reduced surface protonation. This study demonstrated that magnesium modification enhances Cd²⁺ adsorption by enriching the surface functional groups, lowering pH_pzc, and promoting stronger complexation. The combined analysis of surface site types and pH_pzc using NEM provides a novel quantitative approach for understanding biochar-metal interactions.

Key words: biochar, Cd, magnesium modification, non-electrostatic surface complexation model, surface acid-base properties, adsorption, wsheat straw

摘要：

近年来研究改性生物炭有效去除水中重金属成为热门，但使用表面络合模型从微观探讨生物炭对镉的吸附机理鲜见报道。通过MgCl₂与小麦秸秆生物炭（WB）共热解制备镁改性生物炭（MgWB）。SEM-EDS、FTIR和BET等表征显示镁成功接枝到生物炭，Mg的质量分数增加了3.97%，生物炭表面新增Mg-O峰且-OH、-C-O峰增强，镁改性未改变小麦秸秆的条状管道主体结构但比表面积降低9.94%。293－313 K的等温吸附曲线表明改性前后生物炭的吸附行为均符合Langmuir模型，MgWB在313 K时最大饱和吸附密度（3.50 μmol·m^-2）是WB的3.7倍。Boehm滴定实验显示改性使总酸性官能团增加13.4%，其中代表高、中、低亲和型官能团的羟基、内酯基、羧基浓度分别增加4.28%、8.92%和0.178%。电位滴定曲线显示镁改性导致生物炭的pH_pzc从9.2降为5.3，不同pH条件下Cd²⁺的吸附边实验中WB和MgWB吸附率达90%所需pH分别为9和6，结果表明吸附平衡所需pH值与pH_pzc有关，且pH_pzc的改变会导致生物炭表面位点类型在不同pH条件下的镉吸附差异。利用非静电表面络合模型NEM、以广义复合法进行拟合，最佳参数计算的位点分布曲线显示WB和MgWB的吸附贡献主要为高亲和型位点，该位点与Cd²⁺的吸附态浓度达到最高值所对应的pH分别为10和7；模型拟合参数pK_a和pK_≡SOCd+表明镁改性导致生物炭表面官能团的质子化能力降低，与Cd²⁺的络合能力增强。镁改性可以丰富生物炭表面官能团，通过增加表面位点浓度和降低pH_pzc有效改善生物炭对重金属的吸附。本研究在使用非静电表面络合模型拟合中将表面位点类型的吸附贡献与pH_pzc结合讨论微观吸附机理，为生物炭-重金属的相互作用提供了一种新的量化手段。

关键词: 生物炭, Cd, 镁改性, 非静电表面络合模型, 表面酸碱性质, 吸附, 小麦秸秆