CSA NewsVolume 58, Issue 11 p. 24-24 News & Perspective Biochar and Phosphate Adsorption Robert Harter, Robert Harter Professor Emeritus University of New Hampshire, ManchesterSearch for more papers by this author Robert Harter, Robert Harter Professor Emeritus University of New Hampshire, ManchesterSearch for more papers by this author First published: 30 October 2013 https://doi.org/10.2134/csa2013-58-11-9Citations: 1Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume58, Issue11November 2013Pages 24-24 RelatedInformation

Current knowledge of sorption processes in tropical soils is reviewed. Landscapes throughout the tropics are dominated by oxisols which occupy extensive areas of potentially highly productive soils. These soils are dominated by low-activity sesquioxide minerals and clays that have variable charge surfaces. The limited information on tropical soils available suggests that the composition of the ambient soil solution can influence sorption through changes in particle surface-charge density. Thus the observed decrease in sorption in the presence of divalent index cations may be related to the effect of ionic charge on the double-layer thickness which is manifested through a change in surface-charge characteristics. However, much work needs to be done to differentiate the effect of cation charge on surface-charge density from the competitive effect between the index cation and heavy-metal ions for the sorption sites. The effects of inorganic and organic ligands on adsorption of Cd by variable charge surfaces are also reviewed.

Soil solutions contain a variety of low molecular weight dissolved organics from breakdown of plant residues and exudation from plant roots, and complexation of metals by these organic ligands can play an important role in controlling metal solubility. The effect of the organic ligands acetate, citrate, fumate, malonate, maleate, salycylate, succinate, and tartrate on the extractability and sorption of Cd was investigated in a range of Cd-treated soils varying widely in soil chemical characteristics. Experimental parameters were adjusted to investigate the effect of pH and organic ligand concentration on Cd extractability. The soils were also extracted dth NaNO3 of equivalent pH and concentration. The organic ligands always extracted more Cd than did NaNO3, indicating that organic complexation increased the extractability of Cd from soil. Extractable Cd ranged from <0.30 to 1 mu mol kg(-1); the greatest Cd concentration in each soil was released in the presence of maleate. In general, the ability to desorb Cd followed the order: maleate > fumarate = succinate > tartrate > malonate > oxalate > salicyclate when the pH of the extract was between 3.1 and 3.4; in this pH range competition for complexation between the metal ion and ligand anion is greater than the affinity of the metal ion for the soil surface. For all soils and organics, extractable Cd decreased with increasing pH of the ligand solutions. The amount of NaNO3-extractable Cd also decreased.with increasing pH, and the amount desorbed was not detectable at pH values exceeding 5.5, suggesting that at high pH values, Cd is released into soil solution only as metal-organic complexes.

Alfisols, Entisols, Inceptisols, Ultisols, Vertisols, and Oxisols are all commonly found in tropical and subtropical regions receiving more than 500 mm mean annual rainfall. Landscapes throughout the tropics and subtropics are, however, dominated by Oxisols and Ultisols occupying extensive areas of potentially highly productive soils. The mineral fractions of these soils consist primarily of low-activity clays having variable surface charge that differs from high activity clays in the origin of that charge. Low activity clays are dominated by iron (Fe) and aluminium (Al) oxyhydroxides and 1:1 layer silicates (kaolin). Much research has been conducted on the effects of pH, organic matter (OM), and cation composition of the soil solution on the surface charge characteristics of variable charge soils from the tropics. In general, net negative surface charge increases with increasing soil pH and OM content. Adsorption of metal ions by variable charge soils and minerals also increases as their pH, clay, and OM contents increase. Although the precise mechanisms for the change in net negative charge of soil and mineral surfaces with increasing pH are not fully understood, the generation of negative charge either through dissociation of H+ ions from surfaces or consumption of OH- ions by soils is generally accepted. In soils dominated by permanent charge surfaces, heavy metals are not mobile but in variable charge soils, the low surface charge density creates conditions conducive to increased mobility. Consequently, the adsorption of heavy metals, in particular, cadmium (Cd) by strongly weathered soils in relation to the effects of inorganic and organic ligands and the implications for metal transport are reviewed.

Solution ionic composition strongly affects ion sorption onto solid surfaces. This study evaluated the effect of competing ions on heavy-metal sorption by soil. Investigations were conducted on Ca-saturated soil in 0.5 mmol L-1 CaCl2 solution. Sorption of Ni2+, Co2+, and Cu2+ in binary metal-Ca and ternary Ni-Co-Ca or Ni-Cu-Ca systems were evaluated. Sorption occurred in the order Cu > Ni almost-equal-to Co, with the soil exhibiting selectivity to Cu2+, but not to Ni2+ or Co2+. Nickel sorption was equivalent to Ca2+ release, but neither Co2+ nor Cu2+ sorption was accompanied by an equivalent Ca2+ release. In the ternary systems, neither Co2+ nor 0.016 mmol L-1 Cu2+ caused a significant decrease in Ni2+ sorption. At 0.079 mmol L-1, Cu2+ did cause an observable decrease in Ni2+ sorption. Copper sorption was not affected by Ni2+, but Co2+ sorption decreased in direct proportion to Ni2+ sorption. Apparently, Ni2+ was retained by an exchange mechanism, but Co2+ and Cu2+ sorption was more complex. Nickel competed with Co2+ for sorption sites to a greater extent than did Co2+ for Ni2+ sites. Copper was selectively sorbed from the Ca2+ solution, while Co2+ and Ni2+ were able to compete with Ca2+ only in very dilute solutions. Maintaining a near-neutral soil pH may not always be the best strategy for minimizing heavy-metal mobility. Some ions appear unable to compete with Ca2+ for sorption sites at ionic strengths representative of near-neutral pH. Thus, while absolute solubility may be controlled, mobility in solution may be enhanced by increased ionic strength as the soil pH approaches neutrality.

Kinetic techniques are increasingly being used to characterize soil sorption/desorption processes and results of such studies are being used as sorption model input. There are benefits and limitations to the approach, and to avoid misuse of kinetics researchers should be aware of both. The initial choice among the many techniques should be based on appropriateness of the technique for modeling a process within the soil system. Without such basis, it is more difficult to develop a modeling strategy. Given an appropriate model, empirical data such as rate of sorption and reaction half-times and calculated information such as rate constants and thermodynamic quantities are assessable. In interfacing data from kinetics studies with models, one must always remember, however, that the heterogeneous nature of soils makes proper assignment of sorption mechanisms tenuous. It appears that the rate-determining processes during metal sorption by soil may be exchange reactions for the first few minutes then intraparticular diffusion until an equilibrium is established, but complete characterization of soil sorption kinetics is not so easily attained. For example, the effects of certain quantities (e.g., temperature) commonly varied in kinetics experiments are not always attributable to the sorption reaction itself, but may also alter the sorbent. Given these constraints, it is possible to make some tentative mechanism assignments and to calculate apparent rate coefficients for the reactions.

Kinetics of copper desorption from the A horizon of Paxton soil was investigated following addition of Cu at the rates of 1.6, 3.9, 6.3, and 7.9 µmol Cu/g soil (100, 250, 400, and 500 µg Cu/g soil). The Cu was allowed to react with the soil for 30 min to 4 d, at which time Na‐citrate was added in a 1:1 Cu/citrate charge ratio. Copper adsorption pH was maintained at 5.6, and desorption occurred at pH 6.0. From the desorption kinetics, it appeared that Cu was initially retained on two types of sites. Up to about 1.6 µmol Cu/g was preferentially retained on high energy sites. If Cu addition exceeded this amount, the excess was retained on low energy sites. Within about 24 h, a portion of the Cu initially bound to low energy sites shifted to sites intermediate in energy between the initially observed low energy and high energy sites.

AbstractSoils undergo substantial physical, chemical, and biological changes during the process of drying. Because soils are usually dried before their use in the laboratory, it is important to understand these changes if laboratory results are to be related to field conditions. The effects of air‐drying (25°C) and over‐drying (105°C) moist soils and the effects of remoistening air‐dried and stored soils on copper adsorption are presented. Results indicate that oven‐dried soils adsorbed 3 to 10% less copper than air‐dried soils, which adsorbed 10% less than moist field soils. Remoistened samples, which were incubated for 1 month under aerated conditions, adsorbed an average of 12% more copper than air‐dried and stored soils. Remoistened soils, which were incubated without aeration under a layer of water, showed no significant deviation from adsorption onto air‐dried soils.

AbstractWhen the development of the Langmuir adsorption equation is critically examined, it is evident that the equation soil scientists have been using [C/x/m = C/k + I/kb, where C is concentration of adsorbate, x/m is the amount adsorbed per unit weight adsorbent, k is the adsorption maximum, and b is a constant] is in the wrong form. This error is of no great importance when the equation is merely used to obtain a calculated adsorption maximum for comparison to other adsorbent properties. However, it does become important when attempts are made to understand adsorption dynamics and bonding strengths. The commonly reported curvilinear nature of the C/x/m vs. C plots is simply the result of not considering the effect of desorbed ions in the equilibrium solution, rather than being due to multiple adsorption mechanisms. When the equation is corrected by considering desorbed ions, the isotherm becomes linear. In addition, the constant, b, of the Langmuir equation is not simply related to the bonding energy of the adsorbed ion, but to the ratio of adsorbed and desorbed ion bonding energies.

AbstractThe sensitivity of the Calvet Microcalorimeter makes feasible the measurement of very small heats of reaction. This capability is particularly useful when studying adsorption reactions at solid‐solution interfaces. The instrument must, however, be specially adapted for measurements of this type, since it contains no provision for equilibration and mixing of separate solutions. Previously developed adaptations of the instrument are not satisfactory, because they either do not stir the combined solutions adequately to overcome flocculation problems or their mechanical energy input is high. An instrument has been developed whereby two solutions can be equilibrated in the calorimeter cell, then mixed and stirred with a net mechanical energy input of −2 ± 0.4 mcalories. This instrument, therefore, makes possible the precise measurement of very small heats of reaction.

Two proteins, lysozyme and ovalbumin, were adsorbed to smectite homoionic to first five elements of the alkali and the alkali earth series, hydrogen, aluminum, and lanthanum. Lysozyme was also adsorbed to Na-smectite in the presence of 10−3 to 2.0 N NaCl solutions. Lysozyme adsorption by all clays was of the high affinity type, with 93 percent of the variance in the adsorption maximum being accounted for by the equation Amax = 290.2–5.5 (valence) - 96.8 (ionic radius) - 114.3 (Pauling electronegativity) - 6.5 (ionization potential). Adsorption of lysozyme by Na-smectite was decreased when the reaction occurred in solution with NaCl concentrations of 0.5 N or above. Adsorption of ovalbumin by the smectites was of the constant partition type and pH dependent. All homoionic clays except those saturated with H and Be adsorbed similar amounts of ovalbumin. Attempts to desorb the two proteins were generally unsuccessful, indicating that stabile complexes had been formed.

Soils representing six great groups were collected from 11 states and Puerto Rico. These soils were selected to provide a broad range in characteristics normally associated with phosphorus adsorption. Regression analysis indicated that aluminum and organic matter in the soil are primarily responsible for phosphorus adsorption. The pH 4.8 NH4OAc extractable aluminum and HCl-NaOH extractable aluminum were particularly useful in accounting for phosphorus adsorption. From the data obtained, it appears that iron probably occupies a secondary role in phosphorus adsorption by soil, becoming important only in local regions where aluminum and/or organic matter content is relatively constant.

SynopsisIncubation of a Toledo clay loam soil revealed that as soil water is increased above field‐capacity the soil environment is changed from oxidizing to reducing conditions. This transformation occurred in about five days. Thus, a soaking rain could create an healthy environment for plant growth, unless drainage is prompt. The formation of the genetic soil profile can also be partially explained by this rapid reduction.