The Variation of Heavy Metals Bioavailability in Sediments of Liujiang River Basin, SW China Associated to Their Speciations and Environmental Fluctuations, a Field Study in Typical Karstic River

Abstract

The bioavailability of heavy metals (HMs) in sediments is closely related to the security of the aquatic environment, but their impacts are poorly researched, particularly in karstic rivers. Therefore, Liujiang River Basin was taken as an example in this study. Seven HMs were analyzed to determine the bioavailability and speciations of HMs in sediments. Moreover, the impacts of environmental factors on HMs were identified. The obtained results suggested that HMs in the sediments are all within their permissible exposure limit (PEL), but Cd and Zn are significantly higher than the soil baseline. Most HMs were found to be in a residual fraction, while their exchangeable fraction was found to be in an extremely low ratio. HMs in bioavailable parts are significantly higher than in the exchangeable and carbonate-bound phases but lower than in the non-residual phase, which demonstrated that HM bioavailability is not confined to the exchangeable and carbonate-bound phases. The correlation coefficients commonly decreased with decreasing speciation ratios, which suggested that the overall bioavailability of metals should be determined by speciation ratios instead of speciations themselves. Noteworthily, most HMs in the residual form were found to be significantly correlated with their overall bioavailability, which highlighted the potential bioavailability of residual form. The non-correlations between pH, electrical conductivity (EC), total dissolved solids (TDS), and HM bioavailability suggested that HMs in the carbonate-bound phase are stable and unsusceptible to environmental variations, while the significant correlations between redox potential (Eh), turbidity, organic matter (OM), main grain size (Mz), and HM bioavailability suggested that HMs in the reducible and oxidizable forms are susceptible to environmental fluctuations. Therefore, the variation of HM bioavailability in karstic rivers is largely regulated by their reducible and oxidizable forms instead of their carbonate-bound form.

The Accumulation and Transformation of Heavy Metals in Sediments of Liujiang River Basin in Southern China and Their Threatening on Water Security

“…The more significant correlations of HMs can be only found with Eh and turbidity rather than other hydrochemical factors, suggesting that Eh and suspended particles should be treated as the manipulators of HMs accumulation in rivers. It is well-known that higher Eh promotes the formation of Fe/Mn oxides in sediments, which will aggravate the aggregation of exogenous HMs [32]. That explained the positive significant correlation between Zn, Pb, Cd and Eh.…”

“…However, with the shield of the carbonate ions in karst river [30], exogenous metals did not appear in water with large amounts, which confirmed by the significantly low content of metals within threshold in water [31]. On the contrary, the high ratio of metals in non-residual form reported formerly, suggested exogenous metals could largely hidden in sediments [32]. Despite the composition of metals speciations in sediments were investigated formerly in Liujiang River, the interactions were only referred to be between metals bioavailability and their chemical forms [30,32].…”

“…The variation of Eh is 94.89-161.10 mV, while the variation of EC and TDS is 141.5-252.2 and 71.1-127.2, respectively. Turbidity is an index of suspended particulate matter in water, which ranges from 8.13-28.10 NTU, which is higher downstream of the Liujiang River Basin (Refer to Hao, 2021) [32]. should be blamed on the variation of particulate sources and the sedimentary environment.…”

“…In order to further understand the specific mechanism of HM accumulation in sediments, the correlations analysis between different forms of HMs and various factors are shown in Table S6. In general, a large input of exogenous HMs can not only change the influences of hydrochemical fluctuations on the speciation of HMs in sediments, but also can reverse the influences of hydrochemical fluctuations on the speciation distribution of HMs in sediments [32], which is well-reflected in Cd. The increase of EC did not aggravate the competition of ions to reduce the content of Cd in the non-residual phase [22], but intensified the accumulation of Cd in all non-residual phases, which directly elevates the mobility and bioavailability of Cd.…”

“…On the contrary, the high ratio of metals in non-residual form reported formerly, suggested exogenous metals could largely hidden in sediments [32]. Despite the composition of metals speciations in sediments were investigated formerly in Liujiang River, the interactions were only referred to be between metals bioavailability and their chemical forms [30,32]. Their interactions on metals accumulation and conversion have not been identified.…”