Shiqiang Wei

Preliminary investigation of phosphorus adsorption onto two types of iron oxide-organic matter complexes

Iron oxide (FeO) coated by natural organic matter (NOM) is ubiquitous. The associations of minerals with organic matter (OM) significantly changes their surface properties and reactivity, and thus affect the environmental fate of pollutants, including nutrients (e.g., phosphorus (P)). In this study, ferrihydrite/goethite-humic acid (FH/GE-HA) complexes were prepared and their adsorption characteristics on P at various pH and ionic strength were investigated. The results indicated that the FeO-OM complexes showed a decreased P adsorption capacity in comparison with bare FeO. The maximum adsorption capacity (Qmax) decreased in the order of FH (22.17 mg/g)>FH-HA (5.43 mg/g)>GE (4.67 mg/g)>GE-HA (3.27 mg/g). After coating with HA, the amorphous FH-HA complex still showed higher P adsorption than the crystalline GE-HA complex. The decreased P adsorption observed might be attributed to changes of the FeO surface charges caused by OM association. The dependence of P adsorption on the specific surface area of adsorbents suggests that the FeO component in the complexes is still the main contributor for the adsorption surfaces. The P adsorptions on FeO-HA complexes decreased with increasing initial pH or decreasing initial ionic strength. A strong dependence of P adsorption on ionic strength and pH may demonstrate that outer-sphere complexes between the OM component on the surface and P possibly coexist with inner-sphere surface complexes between the FeO component and P. Therefore, previous over-emphasis on the contributions of original minerals to P immobilization possibly over-estimates the P loading capacity of soils, especially in humic-rich areas.

Application of polyacrylamide to reduce phosphorus losses from a Chinese purple soil: a laboratory and field investigation.

Use of anionic polyacrylamide (PAM) to control phosphorus (P) losses from a Chinese purple soil was studied in both a laboratory soil column experiment and a field plot experiment on a steep slope (27%). Treatments in the column study were a control, and PAM mixed uniformly into the soil at rates of 0.02, 0.05, 0.08, 0.10, and 0.20%. We found that PAM had an important inhibitory effect on vertical P transport in the soil columns, with the 0.20% PAM treatment having the greatest significant reduction in leachate soluble P concentrations and losses resulting from nine leaching periods. Field experiments were conducted on 5m wide by 21m long natural rainfall plots, that allowed collection of both surface runoff and subsurface drainage water. Wheat was planted and grown on all plots with typical fertilizer applied. Treatments included a control, dry PAM at 3.9 kg ha(-1), dry PAM at 3.9 kg ha(-1) applied together with lime (CaCO(3) at 4.9 t ha(-1)), and dry PAM at 3.9 kg ha(-1) applied together with gypsum (CaSO(4).2H(2)O at 4 t ha(-1)). Results from the field plot experiment in which 5 rainfall events resulted in measurable runoff and leachate showed that all PAM treatments significantly reduced runoff volume and total P losses in surface runoff compared to the control. The PAM treatments also all significantly reduced water volume leached to the tile drain. However, total P losses in the leachate water were not significantly different due to the treatments, perhaps due to the low PAM soil surface application rate and/or high experimental variability. The PAM alone treatment resulted in the greatest wheat growth as indicated by the plant growth indexes of wheat plant height, leaf length, leaf width, grain number per head, and dried grain mass. Growth indexes of the PAM with Calcium treatments were significantly lesser. These results indicate that the selection and use of soil amendments need to be carefully determined based upon the most important management goal at a particular site (runoff/nutrient loss control, enhanced plant growth, or a combination).

Adsorption of Arsenate on Iron Oxides as Influenced by Humic Acids.

Humic acid (HA) and iron oxides (FeOs) commonly coexist, and their interactions alter their ability to adsorb pollutants in the environment. The influences of HA on arsenate [As(V)] adsorption on FeOs were investigated on the preformation of complexes and their coexistence with As(V) in solution. The results indicated that HA could be strongly adsorbed on FeOs, and the adsorption capacity (, mg g HA-C) followed the sequence goethite (7.73) > ferrihydrite (3.14) > hematite (2.25) with a desorption rate <1%. The HA adsorbed existed uniformly on the FeOs surfaces in spot form and did not change the x-ray diffraction (XRD) patterns of FeOs. The formation of FeOs-HA complexes altered As(V) adsorption with a reduced adsorption capacity, prolonged reaction kinetics, and enhanced adsorption strength. The As(V) adsorption on both FeOs and FeOs-HA complexes decreased with increasing pH (2.5-9) or decreasing ionic strength (0.2-0 M). The coexistence of HA in solution linearly decreased the As(V) adsorption on FeOs. Thus, our results demonstrated two impact pathways of HA on As(V) adsorption on FeOs: (i) blockage or occupation in the surface sites of FeOs if HA preformed complexes with FeOs and (ii) a competition to the surface sites of FeOs when HA coexisted with As(V) in the solution.

Pb (II) bioavailability to algae (Chlorella pyrenoidosa) in relation to its complexation with humic acids of different molecular weight

Humic acid (HA) has a major influence on the environmental fate of metal ions due to its heterogeneity in chemical compositions, structure and functional groups. In this study, we investigated the effect of humic acid (HA) with different molecular weight (Mw) on the bioavailability of Pb for a representative algae-Chlorella pyrenoidosa. The results showed that HA with larger Mw had stronger inhibitory effects on the bioavailability of Pb to algae, and the biosorption capacity of Pb decreased with increasing Mw, which is in accordance with the variations of complexation capacities of Pb for HA fraction. In addition, we found that HA with Mw lower than 10 kDa could increase the biosorption capacity of Pb. The considerable differences among the Mw fractions on Pb biosorption were mainly attributed to their properties and corresponding complexation capacities. Phenolic groups were responsible for the variations of binding capacities among different Mw fractions, and it could also better explain the bioaccumulation of Pb to the membranes of algae. By using NICA-Donnan model, we found that over 60% of Pb ions were bound by HAs through specific binding, and the formation of Pb-HAs complex were non-bioavailable to algae, which was proved by the considerably decreasing percentage of internalized Pb. This study provided further insight into the bioavailability of Pb to algae as influenced by the complexation of HA with metal ion such as Pb.

Relating Cd 2+ binding by humic acids to molecular weight: A modeling and spectroscopic study

Molecular weight (Mw) is a fundamental property of humic acids (HAs), which considerably affect the mobility and speciation of heavy metals in the environment. In this study, soil humic acid (HA) extracted from Jinyun Mountain, Chongqing was ultra-filtered into four fractions according to the molecular weight, and their properties were characterized. Complexation of cadmium was investigated by titration experiments. For the first time, Langmuir and non-ideal competitive adsorption-Donna (NICA-Donnan) models combined with fluorescence excitation-emission matrix (EEM) quenching were employed to elucidate the binding characteristics of individual Mw fractions of HA. The results showed that the concentration of acidic functional groups decreased with increasing Mw, especially the phenolic groups. The humification degree and aliphaticity increased with increasing Mw as indicated by elemental composition analysis and FT-IR spectra. The binding capacity of Cd2+ to Mw fractions of HA followed the order UF1 (<5kDa)>UF2 (5-10kDa)>UF4 (>30kDa)>UF3 (10-30kDa). Moreover, the distribution of cadmium speciation indicated that the phenolic groups were responsible for the variations in binding of Cd2+ among different Mw fractions. The results of fluorescence quenching illustrated that the binding capacity of Cd2+ to Mw fractions was controlled by the content of functional groups, while the binding affinity was largely influenced by structural factors. The results provide a better understanding of the roles that different HA Mw fractions play in heavy metal binding, which has important implications in the control of heavy metal migration and bio-toxicity.

Determination and characterization on the capacity of humic acid for the reduction of divalent mercury

Reduction capacity (RC) has been recognized as an important parameter for evaluating the redox role of humic acid (HA). Thus, for understanding the capacity of HA for the reduction of mercury (Hg2+), chemical reduction capacity (CRC), microbial reduction capacity (MRC) and native reduction capacity (NRC) for mercury reduction by three types of HAs extracted from various sources (SH, TJ and JY) were measured respectively, following the pre-treatment of the HA samples by saturated hydrogen oscillation, soil solution incubation and the control (without any pretreatment). Three electron acceptors including mercuric chloride (HgCl2), mercuric nitrate [Hg(NO3)3] and ferric citrate (FeCit) as a reference were adapted respectively based on the Fe3+ reduction method. The principal results indicated that: (1) the capacity of HA for the reduction of Hg was significantly affected by various electron acceptors, with the RC values obtained under FeCit condition were all greatly higher than those in the conditions of Hg(NO3)2 and HgCl2, which suggested that the RC obtained using Fe3+ reduction method could exaggerate the actual capacity of HA for the reduction of Hg2+; (2) significant differences existed for the reduction capacity of Hg2+ by different HAs, with those of JY were the highest, which were (0.95 +/- 0.03) mmol(c) x mol(-1) (NRC), (5.95 +/- 0.63) mmol(c) x mol(-1) (CRC) and (6.26 +/- 0.51) mmol(c) x mol(-1) (MRC) respectively; (3) HA in solution forms had approximately 100% - 691.67% higher reduction capacity than those as solid forms. Meanwhile, through comparison of the differences among three RC indices, higher CRC and MRC values than NRC were observed, but no significant difference between CRC and MRC was concluded. Thus, CRC may not be applicable to comprehensively represent the real reduction capacity of HA for Hg reduction under microbial condition.