铁改性生物炭对磷的吸附及磷形态的变化特征

Phosphorus Adsorption by and Forms in Fe-modified Biochar

利用农作物残体小麦秸秆为原料制备生物炭,并用氯化铁溶液改性,考察了改性后生物炭元素组成和表面官能团的变化、改性和吸附后生物炭中磷形态变化特征以及溶液初始pH的影响,分析了铁改性生物炭对磷的吸附机理。结果表明,生物炭经氯化铁溶液改性后C的质量分数大幅下降,O和Fe的质量分数大幅上升;表面的羧基含量增加,碱性官能团含量显著降低。铁改性使生物炭对磷的吸附平衡时间由改性前的60 min增至120 min;铁改性后,生物炭的理论最大吸附量为10.1 mg·g-1,是改性前的19.4倍。改性前生物炭对磷的吸附主要是物理吸附,表现为交换态磷含量大幅增加,占吸附总量的82.1%;而铁改性生物炭主要是化学吸附,表现为铁结合态磷的增幅最大,占吸附总量的66.7%,交换态磷仅占26.6%。随溶液初始pH的增加,铁改性生物炭对磷的去除率先增加后下降,pH=7时去除率最高,去除率随pH的变化与交换态磷含量密切相关;随着pH升高,铁结合态磷有向闭蓄态磷转化的趋势。

Biochar has great potential to adsorb phosphorus, and its phosphorus forms are important for phosphorus release and bioavailability. In this study, wheat straw derived biochar was used to examine the changes of elements and surface functional groups, phosphorus adsorption and forms in biochar after modification. Modification obviously decreased C mass fraction, but increased O and Fe mass fractions in the biochar. The content of carboxyl in biochar significantly increased, while basic functional groups decreased after modification. Modification led to the equilibrium time of phosphorus adsorption by biochar increasing from 60 min to 120 min. The maximum adsorption capacity of phosphorus by Fe-modified biochar was 10.1 mg·g^-1, which was 19.4 times that of unmodified biochar. The content of exchangeable phosphorus in unmodified biochar accounted for 82.1% of the total adsorption, suggesting that the adsorption mechanism was mainly physical adsorption. However, the content of iron-bound phosphorus in Fe-modified biochar was 66.7% of the total adsorption, while exchangeable phosphorus only 26.6%, implying that phosphorus was adsorbed by chemical adsorption. Increasing initial pH enhanced the removal percentage of phosphorus by Fe-modified biochar until pH7, and then decreased. The pH dependence of phosphorus removal rates was closely related to the content of exchangeable phosphorus. Increased pH might promote iron-bound phosphorus transform into occluded phosphorus.