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Background. Wastewater treatment using physical, chemical, and biological methods is primary solution for the reduction of water pollution that reaching the critical thresholds. The members of subfamily Lemnoideae, commonly called duckweed, are considered the most efficient aquatic plants for wastewater remediation. Although properties of duckweed to survive in water with high concentration of heavy metal ions such as chromium, cobalt, lead, nickel and cuprum are well documented, the growth of duckweed in water with high concentrations of manganese and the efficiency of retention of manganese from water by these species has not been estimated. Objective. Four duckweed species (Spirodela polyrhiza, Landoltia punctata, Lemna aequinoctialis, and L. turionifera) were used for establishment of influence Mn on their vitality and growth and for studying their potential for phytoremediation of fresh water with elevated manganese concentration. Methods. Duckweed collected in Eastern China was introduced in tissue culture in vitro by surface sterilization. The identification of the collected duckweed species was determined by DNA barcoding using primers specific for chloroplast intergenic spacers atpF-atpH (ATP) and psbK-psbL (PSB). The experiments for establishment of influence Mn on duckweed growth carried out in aseptic condition. To determinate concentration of Mn, the samples of different water type (Hongze Lake, ponds around Hongze Lake, Huaian local municipal sewage plant and industrial sewage plant) were analyzed by the Inductively Coupled Plasma Optical Emission Spectrometry. Results. The most sensitive duckweed to Mn was S. polyzhiza, the first characteristic symptoms of toxicity like brown spots have appeared when concentration of Mn was 40 mg/L, the concentration 200 mg/L Mn resulted in chlorosis and death of fronds. L. aequinoctialis and L. turionifera had similar effects in SH medium supplemented with 650 mg/L and 975 mg/L Mn, respectively. L. punctata was the most tolerant duckweed to Mn plants continued to grow even at concentration 975 mg/L. Response of duckweed on Mn was dependent on availability of nitrogen in nutrient medium. Using four duckweed species for treatment of water containing 4.12 mg/L Mn allowed to reduce concentration until safe level of standard (0.1 mg/L Mn). Conclusions. All investigated duckweed species (S. polyrhiza, L. punctata, L. aequinoctialis, and L. turionifera) were characterized by a high level of resistance to manganese, especially L. punctata. Response of duckweed on Mn was dependent on availability of nitrogen in nutrient medium. The tested species of subfamily Lemnoideae were high effective for phytoremediation of water with elevated manganese concentration.

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Abstract It is of great significance to remove Cr(VI) from water as a result of its high toxicity. Biochar from corn straw was modified by different acids (HNO3, H2SO4 and H3PO4) to remove Cr(VI) from aqueous solution. To estimate the removal mechanisms of Cr(VI) by the acid-modified biochars, batch experiments were performed in the light of contact time, Cr(VI) concentration, and pH, and the characteristics of acid-modified biochars before and after Cr(VI) adsorption were investigated by fourier transform infrared spectra (FTIR) and X-ray photoelectron spectroscopy (XPS). The adsorption kinetics of Cr(VI) by acid-modified biochars were consistent with the pseudo-second-order model, and the adsorption isotherm obeyed the Freundlich model. Furthermore, the acid- modified biochars could supply more oxygen-containing functional groups (-COOH and -OH) as electron donor (e−) and hydrogen ion (H+) to enhance the reduction of Cr(VI) to Cr(III), resulting in enhanced removal of Cr(VI). HNO3-modified biochar exhibited the highest removal efficiency of Cr(VI). In general, the acid modifition of biochar was an effective method to increase the removal of Cr(VI).

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