Effect of humic substances quality on chemical speciation and stability of heavy metals in smelter area soils

Abstract

(구)장항제련소 지역은 오랜 제련활동으로 인한 비소, 구리, 납 등의 중금속 오염이 문제시 되어 왔다. 이에 본 연구에서는 토양 내 중금속의 이동성을 감소시키기 위하여 중금속 킬레이팅 능력이 뛰어난 펄빅산 (fulvic acid

FA)과 복합체의 안정도가 우수한 휴믹산 (humic acid

HA)의 비율을 달리하여 (HA/FA=2, HA/FA=1, HA/FA=0.5, 대조군) 토양에 처리한 후 90일 간 항온배양실험을 수행하여 화학종 변화와 안정도상수를 확인하였다. 실험 결과, 비소는 펄빅산과 복합체를 형성할 때 안정한 반면, 구리와 납은 휴믹산과 복합체를 형성할 때 더 안정했다. 대조군을 포함한 모든 토양이 산성화되었지만, 휴믹물질의 첨가로 인하여 토양의 산성화가 감소되었다. 이로 인하여 구리와 납은 대조군에 비하여 이동성이 적은 형태(철-망간 산화물 및 유기물 결합 분획)가 증가하였으며, 가용성 형태(수용성 및 교환성, 탄산염 결합 분획)가 감소하였다. 또한 휴믹산의 비율이 높을수록 이동성 분획의 감소와 안정한 형태의 증가가 가장 효과적임을 확인하였다. 하지만 비소는 대조군과 처리군 사이에서 유의적인 차이를 보이지 않았으며, 비소가 배양이 진행됨에 따라 안정화된 것은 토양 산성화의 영향으로 보인다. 중금속은 휴믹산보다 펄빅산과 더 많이 결합하였으며, 비소와 구리의 안정도상수는 휴믹산의 비율이 높을수록, 납의 경우 펄빅산의 비율이 높을수록 안정도상수가 커졌다. 본 연구 결과를 통하여 중금속으로 오염된 토양에서 휴믹물질이 중금속을 부동화하고 안정화하는데 기여할 수 있으며, 이들을 활용할 때 중금속과 휴믹물질의 특성이 고려되어야 함을 확인하였다.

Heavy metal contamination in soils such as smelter area soils has been a worldwide problem. As one of the remediation technologies, application of humic substances can decrease the availability and mobility of metals by increasing partitioning of metals into less mobile forms. Different quality of humic substances, which is the ratio of humic acid (HA) to fulvic acid (FA), is expected to influence the ability to chelate metals and the stability of humic substance-metal complexes. The ion-exchange method by Schnitzer and Skinner (1965) was employed to measure the stability constants and different metal-humic substance complexes showed variable stability constant (log K) values (As-HA: 2.64, As-FA: 3.71, Cu-HA: 7.21, Cu-FA: 3.24, Pb-HA: 8.26, Pb-FA: 8.22, respectively). As-FA complex was more stable than As-HA complex while copper and lead complexes with HA were more stable than those with FA. Different quality of humic substances (HA/FA=2, HA/FA=1,HA/FA=0.5, and control) was treated to the contaminated soil and time-variant chemical speciation of three toxic heavy metals (As, Cu, and Pb) was performed for 90 days. Sequential extraction technique (Tessier et al., 1979) was employed to fractionate metals and organo-metallic extraction method (Donisa et al., 2003

Podlešáková et al., 1992) was used to specify the treatment effect of humic substances. The addition of humic substances lessened the decrease of soil pH. Accordingly, in cases of Cu and Pb, the addition of humic substances increased less mobile chemical forms (carbonate bound, Fe-Mn oxide bound, and organic bound fractions) but decreased available forms (water soluble and exchangeable fraction) than control group. In particular, HA/FA=2 treatment was most effective among treatment groups to decrease the amount of mobile fractions and increase the stable forms. However, As did not show any significant differences between control and treatments and its chemical speciation shift to stable forms might have been attributed to the effect of soil acidification. Most metals were distributed to the fulvic fractions, which meant fulvic acids had greater chelating ability than humic acids did. However, in terms of stability constants, the log K values for metal-HA complexes were inclined to increase while the log K values for metal-FA complexes decreased. The results of the study suggested that humic substances can contribute to the immobilization and stabilization of metals in heavily polluted soils.