Review on the degradation of chlorinated hydrocarbons by persulfate activated with zero-valent iron-based materials.

Chlorinated hydrocarbons (CHCs) are often used in industrial processes, and they have been found in groundwater with increasing frequency in recent years. Several typical CHCs, including trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), carbon tetrachloride (CT), etc., have strong cytotoxicity and carcinogenicity, posing a serious threat to human health and ecological environment. Advanced persulfate (PS) oxidation technology based on nano zero-valent iron (nZVI) has become a research hotspot for CHCs degradation in recent years. However, nZVI is easily oxidized to form the surface passivation layer and prone to aggregation in practical application, which significantly reduces the activation efficiency of PS. In order to solve this problem, various nZVI modification solutions have been proposed. This review systematically summarizes four commonly used modification methods of nZVI, and the theoretical mechanisms of PS activated by primitive and modified nZVI. Besides, the influencing factors in the engineering application process are discussed. In addition, the controversial views on which of the two (SO4·- and ·OH) is dominant in the nZVI/PS system are summarized. Generally, SO4·- predominates in acidic conditions while ·OH prefers neutral and alkaline environments. Finally, challenges and prospects for practical application of CHCs removal by nZVI-based materials activating PS are also analyzed.

Synthetic Humic Acids Solubilize Otherwise Insoluble Phosphates to Improve Soil Fertility.

Artificial humic acids (A-HA) made from biomass in a hydrothermal process turn otherwise highly insoluble phosphates (e.g. iron phosphate as a model) into highly available phosphorus, which contributes to the fertility of soils and the coupled plant growth. A detailed electron microscopy study revealed etching of the primary iron phosphate crystals by the -COOH and phenolic groups of humic acids, but also illustrated the importance of the redox properties of humic matter on the nanoscale. The combined effects result in the formation of then bioavailable phosphate nanoparticles stabilized by humic matter. Typical agricultural chemical tests indicate that the content of total P and directly plant-available P improved largely. Comparative pot planting experiments before and after treatment of phosphates with A-HA demonstrate significantly enhanced plant growth, as quantified in higher aboveground and belowground plant biomass.

Artificial humic substances improve microbial activity for binding CO2.

Humic substances (HS) are an indicator of fertile soils, but more and more soils keep losing there humic matter. This is mostly due to anthropogenic over-cultivation. Artificial humic acid (A-HA) and artificial fulvic acid were synthesized from agricultural litter, with high similarity to natural HS extracted from soil. These samples were added to black soils, and soil activity and nutrients availability were analyzed. The results demonstrate that the content of dissolved organic matter and total organic carbon (TOC) largely increased. The increase in TOC 28 days after addition of A-HA was 21.4 g/kg. This was much higher than the amount of the added A-HA carbon, which was 0.3 g/kg. As a "secondary" benefit, nutrient availability is increased, promoting the growth of plants. Using high-throughput sequencing we revealed that A-HA strongly supports the growth of photosynthetic Rubrivivax gelatinosus, which induced the carbon sequestration. Thus, application of artificial HS shows potential for biologically amplified carbon sequestration within black soils.

Application of kernel extreme learning machine and Kriging model in prediction of heavy metals removal by biochar.

Kernel extreme learning machine (KELM) and Kriging models are proposed to predict biochar adsorption efficiency of heavy metals. Both six popular ions (Pb2+, Cd2+, Zn2+, Cu2+, Ni2+, As3+) and single ion are considered to test the accuracy of KELM and Kriging models. Two ways (data selection and fix output value) are attempted to improve the model fitting accuracy and the best R2 can reach 0.919 (KELM) and 0.980 (Kriging). In addition, stepwise regression and local sensitivity analysis show that adsorption efficiency has strong relationship with pHsolute and T. Moreover, the most sensitive parameters are T, pHH2O, r, C and pHsolute. The accurate KELM and Kriging models identify the most important controlling factors on metal adsorption, and ultimately provide some sort of predictive framework that will be useful in selecting appropriate biochar for particular treatment scenarios. This, in turn, will reduce the number of metal-biochar adsorption experiments needed going forward.

Preparation of montmorillonite modified biochar with various temperatures and their mechanism for Zn ion removal.

Because of its layered structure, excellent adsorption, catalytic and ion exchange properties, strong passivation, and superior adsorption capacity for heavy metals, montmorillonite as biochar modifier was introduced. Montmorillonite modified biochar composites (BC-MNT) were successfully prepared by one-step hydrothermal progress and subsequent pyrolysis for the application of Zn (II) removal from aqueous solution. Herein, our research mainly explored the effect of different hydrothermal method/pyrolysis temperatures on the adsorption properties of montmorillonite-biochar composites, since that temperature can greatly change the surface functional groups and chemical composition and structure of biochar. Primarily, adopting VK analysis method analyzed raw biochar and montmorillonite modified biochar at different temperature condition. For further research, adsorption mechanism was realized by means of SEM-EDS, XRD, FTIR, and XPS, as concluded that electrostatic interaction between the negative charge of the interlayers and Zn (II) and surface complexation took up a dominant role. The kinetic and isothermal adsorption mathematical model fitting were also performed, which was more suitable for pseudo-second order and Freundlich model, suggesting that the limited speed stage was dominated by chemical forces. The optimal preparation temperature (350 °C) and maximum adsorption amount (8.163 mg·g-1) were determined by the adsorption experiment model.

Analog synthesis of artificial humic substances for efficient removal of mercury.

A cost-effective artificial humic substances (humic acid-modified biochar, HA-BCs) is fabricated by using conventional hydrothermal-assisted pyrolysis technique, and then is considered as a promising adsorbent material for removing mercury ions from aqueous solution. Artificial humic acid (A-HA), humic acid-modified biochar (HA-BCs) are analyzed by using SEM, EA, XRD, FTIR, XPS, and BET techniques. The removal efficiency of mercury ions was greater than 95% after reaching the adsorption equilibrium. Meanwhile, the adsorption kinetics coincided with the pseudo-second-order model and the isotherms for mercury ion sorption can be best interpreted using Freundlich isotherm model, with high regression coefficients (R2 = 0.967-0.990). Furthermore, the surface properties of HA-BCs before and after mercury adsorption are compared and evaluated, realizing that the mechanisms of removal of mercury ions on HA-BCs mainly include surface complexation with oxygen/nitrogen functional groups (-OH, -COOH and -NH2) and formation of precipitation with CO32- and OH-. Furthermore, the used HA-BCs can be regenerated via 0.05 mol/L KI solution and the adsorption capacity of mercury still reaches at 32.57 mg/g after four cyclic utilization.

Activation of porous magnetized biochar by artificial humic acid for effective removal of lead ions.

In this paper, we have successfully prepared porous magnetic biochar with excellent surface area and recovery rate using corn stalks (CS) and waste iron (WI) as precursors. Notably, in order to prevent the incorporated iron oxides from blocking the carbon pores, then resulting in a decrease in specific surface area and reducing the removal efficiency of the material, the optimum range of iron ions can be determined to be 0.04-0.06 mol/L according to the effect of the amount of iron on the magnetic biochar recovery rate and Pb2+ removal capacity. Furthermore, as-synthesized artificial humic acid (A-HA) obtained from waste biomass by hydrothermal humification (HTH) technology has abundant functional groups, which can complex with heavy metals and metal oxides. Therefore, A-HA is introduced as an activator to produce novel porous magnetic biochar materials (AHA/Fe3O4-γFe2O3@PBC) with abundant functional groups (i.e., phenolic-OH, -COOH, etc.), providing high dispersibility and stability, further leading to excellent removal performance (Langmuir removal capacity up to 99.82 mg/g) and recyclable performance (removal capacity after 5 removal cycles is 79.04 mg/g). Multiple removal mechanisms have been revealed, including reduction, complexation, and precipitation.

Fabrication of L-cysteine stabilized α-FeOOH nanocomposite on porous hydrophilic biochar as an effective adsorbent for Pb2+ removal.

Lead (Pb) pollution has caused worldwide attention as it can cause hazards to humans and the environment. Chemical properties and structures of the adsorbent greatly influence the Pb2+ removal efficiency. L-cysteine (L-cy) stabilized porous hydrophilic biochar-supported α-FeOOH nanocomposites (L-cy/FeOOH@PHB) are prepared as an efficient adsorbent via a cheap and simple one-step hydrothermal method for removing Pb2+ from aqueous solution. Characterizations of the synthesized L-cy/FeOOH@PHB revealed that the iron particles distributed uniformly on the surface of porous hydrophilic biochar. The equilibrium adsorption capacity of the L-cy/FeOOH@PHB reaches up to 103.04 mg g-1for Pb2+ removal, higher than other typical materials reported preiously. The adsorption kinetics and isotherms were fitted well with the pseudo-second-order model and the Freundlich model, respectively, suggesting chemical adsorption on the heterogeneous surface and pores of L-cy/FeOOH@PHB. The introduction of L-cysteine provides abundant surface N- and S-containing functional groups as active sites for Pb2+ adsorption and also plays an important role in altering the porous structure, distribution of α-FeOOH nanoparticles, affinity of iron species to biochar, and surface functional groups, which determined the performance of the resultant composites. Notably, regeneration experiments show that Pb2+ adsorption capacity still maintains at 77.3 mg g-1 on L-cy/FeOOH@PHB after five successive utilizations, indicating the potential applicability for removing Pb2+ from aqueous solution.

High-dispersion zero-valent iron particles stabilized by artificial humic acid for lead ion removal.

Nano zero-valent iron (nZVI), as a high-efficiency adsorbent for heavy metals, often suffers being oxidized and assembling together due to small size and super reactivity, further decreasing its adsorption performance and limiting application ranges. Herein, we have designed a novel adsorbent with high-dispersion nZVI stabilized by as-prepared artificial humic acid (AHA-nZVI) derived from hydrothermal humification (HTH) technology. Introduction of artificial humic acid (A-HA) can effectively reduce the oxidation and agglomeration of nZVI, leading to superior kinetic removal efficiency of Pb2+ (> 99.2%) and huge Langmuir removal capacity of 649.0 mg/g. The combination of nZVI and A-HA (contained abundant functional groups, i.e. -OH and -COOH) via C-O-Fe bonding makes nZVI have good dispersion and oxidation resistance. Multiple interaction mechanisms including reduction reaction, complexation and co-precipitation between heavy metals and AHA-nZVI samples are realized. Overall, AHA-nZVI is a promising material for high-performance heavy metal contaminated water treatment.

A novel electrochemical modification combined with one-step pyrolysis for preparation of sustainable thorn-like iron-based biochar composites.

A novel method incorporating electrochemical (EC) modification and one-step pyrolysis is developed to prepare sustainable Fe3O4-based magnetic adsorbent (EC-Fe3O4/BC) via pyrolysis of FeCl3-pretreated corn straw-derived biochar under an electric field generated by graphite electrode. Morphological characterization revealed a uniform dispersion of rod-like crystalline Fe3O4 nanoparticles in the inner and outer structure of biochar. The EC modification also introduced more oxygen-containing functional groups, which contributed to an outstanding Pb adsorption capacity (113 mg g-1) and fast kinetics (0.054 g mg-1 h-1). Therefore, the EC modification is a simple and time-saving method to effectively fabricate magnetic biochar adsorbent for high-performance wastewater treatment.

Porous biochar composite assembled with ternary needle-like iron-manganese-sulphur hybrids for high-efficiency lead removal.

Hierarchical porous biochar derived from corn straw containing ternary needle-like iron-manganese-sulphur composites (Fe-Mn-S@HCS) are fabricated, and their physicochemical characteristics and performance for Pb removal were examined in detail. Introduction of Mn (transition metal) into Fe-biochar composites can effectively alter the chemical state of Fe; simultaneous doping with S can enhance cation exchange for Pb removal. High uptake of Pb by Fe-Mn-S@HCS in a short time period was observed with the adsorption capacity of 181.5 mg g-1 and the pseudo-second-order rate constant of 0.075 g mg-1 h-1. Complexation, reduction, and precipitation were found to be involved in the Pb removal by Fe-Mn-S@HCS based on the results of HRTEM, XPS, and XRD analyses. This study demonstrated the feasibility of Fe-Mn-S biochar composites for high-efficiency Pb removal from aqueous solution.