Why a fern (Pteris multifida) dominantly growing on an arsenictheavy metal contamînated soil does not accumulate arsenic?

Abstract
A kind of fern (Pteris multifida) is growing dominantly on a lead/zinc mine tailings contaminated soil located in Shaoxing, Zhejiang Province, the People's Republic of China. The soil was heavily contaminated by heavy metals and arsenic, especially by lead, arsenic, and zinc. The total concentration (mg/kg) ranges for As, Cd, Cr, Cu, Pb, and Zn in the soil were 1370-4539, 3.0-9.3, 39-48, 65-206, 5271-16397, and 387-1221, respectively. The accumulative characteristics of heavy metals and arsenic in the fern pteris multifida showed that it accumulated lead 86-237 mg/kg in leaf, 33-741 mg/kg in stem, 879-4040 mg/kg in root and accumulated arsenic only 0.3-1.6 mg/kg in leaf, 0.5-8.0 mg/kg in stem, 24-45 mg/kg in root. Some ferns were reportedly As hyperaccumulators by some researchers. Why a fern (Pteris multifida) grown on a high As-contaminated soil in the present research could not accumulate arsenic? Further analyses indicated that water-soluble arsenic was almost zero and water-soluble Pb was high. It is suggested that high concentration Pb reacted with arsenic in soil solution to form a stable mineral. X-ray diffraction détermination proved that there was a mineral, beudantite PbFe ₃ (AsO4 ₄)(SO ₄)(OH) ₆) in  <0.002 mm clay fraction of the contaminated soil. The solubility product (Ksp) of beudantite was reportedly 10 ^-21, leading to very low water-soluble As in the soil.