This study prepared iron-manganese oxide-modified biochar (FM-BC) by impregnating rice straw biochar (BC) with a mixed solution of ferric nitrate and potassium permanganate. The effects of pH, FM-BC dosage, interference of coexisting ions, adsorption time, incipient Pb(II) concentration, and temperature on the adsorption of Pb(II) by FM-BC were investigated. Moreover, the Pb(II) adsorption mechanism of FM-BC was analyzed using a series of characterization techniques. The results showed that the Fe-Mn oxide composite modification significantly promoted the physical and chemical functions of the biochar surface and the adsorption capacity of Pb(II). The specific surface area of FM-BC was 18.20 times larger than that of BC, and the maximum Pb(II) adsorption capacity reached 165.88 mg/g. Adsorption kinetic tests showed that the adsorption of Pb(II) by FM-BC was based on the pseudo-second-order kinetic model, which indicated that the adsorption process was mainly governed by chemical adsorption. The isothermal adsorption of Pb(II) by FM-BC conformed to the Langmuir model, indicating that the adsorption process was spontaneous and endothermic. Characterization analyses (Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy) showed that the adsorption mechanism of Pb(II) by FM-BC was mainly via electrostatic adsorption, chemical precipitation, complexation, ion exchange, and the transformation of Mn2O3 into MnO2. Therefore, FM-BC is a promising adsorbent for Pb(II) removal from wastewater.
Phosphorus (P) is an essential nutrient element for crop growth. The effects of P surplus or deficit on Cd absorption and transport in rice in Cd-polluted farmland is not clear. The effects of P deficiency and P sufficiency on Cd uptake, transport, and accumulation in rice under Cd stress were investigated by applying different levels of phosphorus (NaH2PO4) in a hydroponic experiment. The results showed that:① with the increase in ρ(P) (1.5-48.0 mg·L-1), the biomass in all parts of the rice plants had no obvious change, and the contents of photosynthetic pigment (chlorophyll a, chlorophyll b, and carotenoid) firstly ascended and then descended; high concentrations of P inhibited the synthesis of photosynthetic pigments. ② Under Cd stress, when the P was deficient (1.5-6.0 mg·L-1) or sufficient (12.0-48.0 mg·L-1), the Cd content in different parts of the rice increased with the increase in P addition level, and the maximum increase in Cd content in brown rice was 132.1% and 191.2%, respectively. ③ The P/Cd of brown rice showed a piecewise decreasing rule under P deficiency and P sufficiency, and the Cd content in brown rice was significantly negatively correlated with P/Cd (P<0.01). These results indicated that elevating phosphorus concentration when rice was under both the conditions of P deficiency and P sufficiency could promote the uptake and transport of Cd by rice roots under Cd stress, thus increasing the accumulation of Cd in aboveground parts and the risk of excessive Cd in rice.
The removal efficiency of Cu, Cd, and Pb from contaminated soils by Na2S2O3 and the technique of heavy metals separating in Na2S2O3 solution were studied. The results indicated that Na2S2O3 solution could effectively extract the heavy metals in soil and the extractable percentage of metals followed the sequence of CuCdPb. Both Na2S and cation exchange resin could separate Cu, Cd in Na2S2O3 solution with efficiency up to 99%.However, separation heavy metals from Na2S2O3 extraction of the alkali soil, incorporating the exchange resin technique, was better than that by the Na2S precipitation method.
Release characteristics of Cd, Cu and Zn from red soils under acid rain and the related influencing factors were studied and differences in heavy metal release from natural soils and contaminated soils were compared in this paper through a leaching experiment of simulated acid rain for two natural red soils and two contaminated red soils1 The results indicated that in the natural red soils, release of Cd, Cu and Zn increased linearly with increasing leaching amounts of simulated acid rain, and the accumulative amounts of released heavy metals increased with increasing acidity of acid rain, showing a sequence of Zn > Cu > Cd. There existed significant linear relationships between Cu or Zn release and Cd release. In the contaminated soils, however, Cd release increased logarithmically with increasing leaching amounts of simulated acid rain, but Cu release increased still in a linear way. Zn release was a linear patter when weak acid rain was applied, but a logarithmic one when strong acid rain was applied. There were non-linear relationships among releases of Cd, Cu and Zn. The release sequence of heavy metals in the contaminated red soils was Cd > Zn > > Cu. About 26% -76% of external Cd and 11% - 68% of external Zn were released through leaching of acid rain, but more than 99% of external Cu was absorbed by the soils. Meanwhile, accumulative release amounts of these three elements were all significantly logarithmically related to H+ accumulative amounts in the leachates, but only those of Cd and Zn were also logarithmically related to TOC ( total organic carbon) accumulative amounts. The relationships between Cu release and TOC release were a power pattern. For the natural red soils, possibility of acid rain stimulating heavy metal contamination was little due to a small release amount of heavy metals; for the contaminated red soils, however, a great amount of heavy metals especially Cd and Zn were released under acid rain, and contamination with heavy metals in soil-plant systems was greatly possible.
Pot experiments were used to study the differences of Cd uptake and accumulation in double-cropping rice in typical soil types. To analyze the soil availability of Cd (DTPA-Cd) in soils and the Cd accumulation in double-cropping rice at different growth stages of the rice, we conducted pot experiments that selected the yellow clayey soil (paddy soil developed from plate shaley parent materials) and the granitic sandy soil (paddy soil developed from granitic parent materials). Exogenous Cd was added with gradients of 0, 0.5, 1.0, 2.0, 5.0, and 10.0 mg·kg-1. Results showed that, during the rice growth period, the available Cd in the yellow clayey soil was higher than that in the granitic sandy soil, and the difference was significant (P<0.01). This showed that the content of Cd in rice (roots, shoots, leaves, rice shells, and brown rice) increased along with the treatment level and with the extension of the rice growth period. The accumulation characteristics of Cd in rice grains and other tissues of rice indicated differences between two seasons and two soil types, that is, late rice was higher in Cd than was early rice, and reddish yellow clayey soil was higher in Cd than granitic sandy soil. Significant positive linear correlations were found between the effective contents of Cd in soils and those in rice tissues (roots, shoots, leaves, and brown rice). The prediction model of Cd in rice and the characteristic equation for rice accumulation of Cd were applied to calculate the critical values of Cd:0.98 mg·kg-1 for early rice and 0.83 mg·kg-1 for late rice in reddish yellow clayey soil, and 0.86 mg·kg-1 for early rice and 0.56 mg·kg-1 for late rice in granitic sandy soil. These threshold values are higher than the National Standards given in "farmland environmental quality evaluation standards for edible agricultural products (HJ 332-2006)." The soil security threshold values and the soil environmental capacities of the two different parent materials varied greatly; therefore, different environmental quality standards may be formulated and different measures may be needed to control Cd pollution in different parent materials.
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